Selina Concise Solutions for ICSE Class 6 Physics Chapter 5 Light

Synposis

• Light itself is not visible, but in the presence of light other objects become visible.
• Light is defined as the external physical cause that affects the eye to produce the sensation of vision.
• Two types of sources of light are:

1. Natural sources such as sun, stars and jugnu.
2. Artificial sources such as fire, electric lamp, electric tube light, a burning candle, a kerosene lamps heated bodies, etc.

• The bodies which themselves emit light are called luminous bodies. Examples: torch, electric lamps electric tube light, burning candle, kerosene lamp, sun, stars, Jugnu etc.
• The bodies which do not emit light by their own, but they become visible because of the light falling on them from a luminous body, are called non-luminous bodies. Examples: moon, earth, table, book, chair etc.
• A medium which allows the passage of light through it easily, is called a transparent medium. Examples: glass, air, water etc.
• A medium which allows only a small amount of light to pass through it, is called a translucent medium. Examples: ground glass, tracing paper etc.
• A medium which does not allow any light to pass through it, is called an opaque medium. Examples: wood, metals etc.
• Light travels in a straight line path. This is called the rectilinear propagation of light.
• The pin hole camera is a simple application of the rectilinear propagation of light.
• The image (or picture) formed in a pin hole camera is upside down (i.e. inverted). On increasing the distance of screen from the pin hole, the size of image increases.
• The shadow of an opaque object is the dark patch obtained on the screen when that opaque object is placed in the path of light.
• Shadow is formed because light travels in a straight line path.
• The shadow is similar to the shape of the object.
• The part of the shadow where no light reaches from the source is completely dark and is called the umbra.
• The part of the shadow where light reaches from only a portion of the source is partially dark and is called the penumbra.
• There is only umbra in the shadow of an opaque object due to a point source. The umbra is bigger in size than that of the object. The umbra increases in size if the screen is moved away from the object.
• The shadow of an object due to a light source smaller than the object contains an umbra surrounded by a penumbra. The umbra is bigger in size than that of the object. Both the umbra and penumbra increase in size as the screen is moved away from the source.
• The shadow of an opaque object due to a light source bigger than the object contains an umbra (which is much smaller in size than the object) surrounded by a penumbra. The umbra diminishes while the penumbra increases in size if the screen is moved away from the object.
• Lunar and solar eclipses are the examples of formation of shadows in nature.
• A lunar eclipse is caused on a certain full moon night when the earth comes in between the sun and the moon so that the earth casts its shadow on the moon.
• A solar eclipse is caused on a certain new moon’s day when the moon comes in between the sun and the earth so that the moon casts its shadow.

Test yourself

A. Short Answer Questions

1. Write true or false for each statement

(a) The moon is a natural source of light.
Answer: False
The moon does not produce its own light; it only reflects the light of the sun. Therefore, it is classified as a non-luminous body in nature.
Teacher's Tip: Remember that natural sources must actually "generate" light themselves.
Exam Tip: If you see "moon" and "source" together, the answer is almost always false.

(b) The moon is self luminous.
Answer: False
Self-luminous bodies are those that glow by themselves, like the sun or stars. The moon remains dark unless it is illuminated by another light source.
Teacher's Tip: Luminous means "glowing," and the moon is just a reflector.
Exam Tip: Use the term "non-luminous" to describe the moon in descriptive answers.

(c) We can see an object through an opaque medium.
Answer: False
Opaque materials like wood or metal block light rays completely. Because light cannot pass through them, we cannot see what is on the other side.
Teacher's Tip: Think of "Opaque" as a "Wall" that light cannot climb over.
Exam Tip: Differentiate clearly between transparent, translucent, and opaque media.

(d) Light passes through glass.
Answer: True
Glass is a transparent material that allows light rays to pass through it without much obstruction. This is why we use glass for windows and spectacles.
Teacher's Tip: If you can see clearly through it, light is passing through it!
Exam Tip: Specify "transparent glass" as standard glass can sometimes be frosted (translucent).

(e) Light travels in a straight line path.
Answer: True
This fundamental property is known as the rectilinear propagation of light. It explains why we cannot see around corners or through bent tubes.
Teacher's Tip: Think of light as a very disciplined soldier that only walks in perfectly straight lines.
Exam Tip: Use the phrase "rectilinear propagation" to score full marks for this concept.

(f) Image formed in a pin hole camera is real.
Answer: True
A real image is one that can be caught on a screen, which is exactly what happens in a pinhole camera. The light rays physically meet at the screen to form the picture.
Teacher's Tip: Real images are always those you can touch on a screen.
Exam Tip: Remember that real images in a pinhole camera are also inverted (upside down).

(g) The image in a pin hole camera gets blurred if the hole is made bigger.
Answer: True
A larger hole allows too many sets of light rays to enter, causing multiple images to overlap. This overlapping creates a fuzzy or blurred appearance instead of a sharp one.
Teacher's Tip: A "Pin" hole must be as thin as a pin to keep the image sharp!
Exam Tip: Explain that a large hole acts like many small pinholes placed together.

(h) A shadow is formed because light travels in a straight line path.
Answer: True
When an object blocks light, the rays cannot bend around it to fill the space behind. This results in a dark area on the screen which we call a shadow.
Teacher's Tip: Shadows are "Light Absences" caused by straight-line travel.
Exam Tip: This is the primary proof used for the rectilinear propagation of light.

(i) Solar eclipse occurs when the sun comes in between the earth and the moon.
Answer: False
A solar eclipse occurs when the moon moves between the sun and the earth. The moon's shadow then falls on the earth, blocking our view of the sun.
Teacher's Tip: During a Solar eclipse, the Moon is the "Middle Man."
Exam Tip: Memorize the order for Solar Eclipse as S-M-E (Sun, Moon, Earth).

(j) If the shadow of earth falls on the moon, the eclipse is called the lunar eclipse.
Answer: True
This happens on a full moon night when the Earth is perfectly aligned between the Sun and the Moon. The Earth blocks the sunlight, casting its shadow over the moon's surface.
Teacher's Tip: Lunar = Moon; so a lunar eclipse is an "Eclipse of the Moon."
Exam Tip: Note that lunar eclipses only happen on full moon nights.

2. Fill in the blanks

(a) Light gives us the sensation of vision.
Answer: vision.
Our eyes detect light rays and our brain interprets them as images. Without light, even with perfect eyes, we would be in total darkness.
Teacher's Tip: Vision is the "superpower" light gives to our eyes.
Exam Tip: Light is the "cause" and vision is the "effect."

(b) The sun is a natural source of light.
Answer: natural.
The sun is a massive star that generates light through nuclear fusion. It is the primary and most important natural source of light for our planet.
Teacher's Tip: Nature provides "Natural" sources like the Sun and stars.
Exam Tip: Be ready to list "Stars" and "Fireflies" as other natural sources.

(c) A medium through which light cannot pass is called the opaque medium.
Answer: opaque.
Opaque materials absorb or reflect all the light that hits them. This is why you cannot see through a brick wall or a wooden door.
Teacher's Tip: Opaque starts with 'O', just like "Obstruct."
Exam Tip: Wood and metal are the two most common examples of opaque media.

(d) A medium which allows light to pass through it easily is called the transparent medium.
Answer: transparent.
In transparent materials, the particles are arranged so that light rays can travel through with almost no scattering. This results in a clear view of objects on the other side.
Teacher's Tip: Think of "Clear" glass or "Pure" water.
Exam Tip: Transparency allows for both light passage and clear vision.

(e) Moon is a non-luminous body.
Answer: non-luminous.
It behaves like a giant mirror in space, reflecting the sun's rays toward us. Since it has no light-producing engine of its own, it is considered non-luminous.
Teacher's Tip: Most things in your room are non-luminous, including you!
Exam Tip: Use this to explain why the moon changes phases based on its position relative to the sun.

(f) Light travels in a straight line path.
Answer: straight line.
This characteristic is the reason why sharp-edged shadows are formed. If light could bend easily, shadows would be fuzzy or non-existent.
Teacher's Tip: Light is the ultimate "Straight Arrow."
Exam Tip: This property is used to design instruments like the pinhole camera.

(g) In a pin hole camera, the image formed is inverted and real.
Answer: inverted and real.
Because light rays from the top of the object go to the bottom of the screen and vice versa, the image flips. It is real because it is physically projected onto the tracing paper.
Teacher's Tip: Inverted means "Upside Down."
Exam Tip: Always mention both characteristics (inverted and real) for a complete answer.

(h) The darkest portion of a shadow is called the umbra.
Answer: umbra.
This area receives absolutely no light from the source because the object blocks it entirely. It is the central, deepest part of any shadow cast by an extended source.
Teacher's Tip: Think of "Umbra" as the "Ultimate" darkness.
Exam Tip: In a point source, the entire shadow is only umbra.

(i) The less dark portion of a shadow is called the penumbra.
Answer: penumbra.
This region surrounds the umbra and receives light from only some parts of the source. It looks like a fuzzy, greyish border around the dark center.
Teacher's Tip: "P" for Penumbra and "P" for Partial darkness.
Exam Tip: Penumbra only forms when using an "extended" or large light source.

(j) Lunar eclipse occurs when the earth comes in between the moon and the sun.
Answer: earth.
The Earth acts as the obstacle that casts a shadow onto the moon. This alignment blocks the moon's usual glow during a full moon night.
Teacher's Tip: Earth is the "Shadow-Maker" in a lunar eclipse.
Exam Tip: The order is Sun - Earth - Moon.

3. Match the following columns
Column A
(a) Wooden block
(b) Sun
(c) Umbra
(d) Eclipse
(e) Solar eclipse

Column B
(i) new moon’s day
(ii) rectilinear propagation of light
(iii) opaque body
(iv) luminous body
(v) complete dark part
Answer:
(a) Wooden block - (iii) opaque body
(b) Sun - (iv) luminous body
(c) Umbra - (v) complete dark part
(d) Eclipse - (ii) rectilinear propagation of light
(e) Solar eclipse - (i) new moon’s day

4. Select the correct alternative

(i) The natural source of light is
1. candle flame
2. electric lamp
3. sun
4. kerosene lamp
Answer: 3. sun
The sun is a star that exists in nature without human intervention. The other options are man-made devices created to provide light artificially.
Teacher's Tip: If humans made it, it's not natural!
Exam Tip: "Stars" and "Fireflies" are other common natural sources to look for.

(ii) The formation of inverted image in a pin hole camera shows that
1. light enables us to see
2. light travels in a straight line path
3. light can pass through the pin hole
4. light does not pass through the pin hole
Answer: 2. light travels in a straight line path
The inversion happens because the top-most light ray can only enter the hole by travelling down, and the bottom-most ray travels up. This criss-cross happens because light cannot curve into the hole.
Teacher's Tip: Inversion is the "Picture Proof" of straight-line travel.
Exam Tip: Use this to explain the working principle of a pinhole camera.

(iii) The luminous body is
1. a lighted bulb
2. earth
3. moon
4. table
Answer: 1. a lighted bulb
A lighted bulb converts electricity into light energy, emitting its own glow. The earth, moon, and table only become visible when external light hits them.
Teacher's Tip: Luminous = Emitter. Non-luminous = Reflector.
Exam Tip: Make sure the bulb is "lighted"; an unlit bulb is non-luminous!

(iv) Umbra is a region of
1. complete darkness
2. partial darkness
3. complete brightness
4. partial brightness
Answer: 1. complete darkness
The umbra is the core of the shadow where no light rays from the source can reach. It is the area where the blocking object is fully obscuring the light source.
Teacher's Tip: Umbra is the "Dark Heart" of the shadow.
Exam Tip: Don't confuse it with penumbra, which is partial darkness.

(v) Penumbra is a region of
1. complete darkness
2. complete brightness
3. partial brightness
4. none of the above
Answer: 3. partial brightness (or partial darkness)
Because it receives light from only a fraction of the light source, it is neither fully dark nor fully bright. This creates the "fuzzy" edge seen in most shadows.
Teacher's Tip: It's the "Grey Zone" of a shadow.
Exam Tip: Penumbra size increases if the light source is larger than the object.

(vi) Solar eclipse occurs on
1. every new moon’s day
2. certain new moon’s day
3. every full moon’s day
4. certain full moon’s day
Answer: 2. certain new moon’s day
It only happens when the moon's orbit aligns perfectly with the sun and earth. While there is a new moon every month, the tilt of the orbit means they don't always line up for an eclipse.
Teacher's Tip: "Certain" is the key word—it doesn't happen every single month!
Exam Tip: Eclipses require perfect "alignment," not just the right phase.

(vii) Lunar eclipse occurs on
1. every full moon’s night
2. certain full moon’s night
3. every new moon’s day
4. certain new moon’s day
Answer: 2. certain full moon’s night
Similar to solar eclipses, the alignment must be perfect. The Earth's shadow must hit the moon, which only happens on specific full moon nights when orbits intersect.
Teacher's Tip: Think of "Lunar" and "Full Moon" as a pair.
Exam Tip: If the question asks "why not every night," mention the tilt of the moon's orbit.

B. Short/Long Answer Questions

Question 1: What is light ? Define it.
Answer: Light is a form of Energy i. e. The external physical cause that affects our eye to produce the sensation of vision.
It is the invisible energy that travels through space and interacts with objects to make them visible to our senses. Without this energy, the biological mechanism of sight would have nothing to process.
Teacher's Tip: Light is the "Messenger" that brings information about objects to our eyes.
Exam Tip: Always include the phrase "sensation of vision" for a scientific definition.

Question 2: How does light make an object visible ?
Answer: An object becomes visible to us when the light after striking the object reaches our eyes. Light itself is not visible, but light makes objects visible to us.
This process is called reflection. Light bounces off the surface of an object and carries the image of that object into our pupils and onto the retina.
Teacher's Tip: Light is like a ping-pong ball that hits a wall (object) and bounces into your hand (eye).
Exam Tip: Mention that we see non-luminous objects only due to reflected light.

Question 3: Name two natural sources of light.
Answer: Sun, stars, jugnu, firefly.
These sources are part of the natural world and produce light through biological or physical reactions. They do not require any human-made technology to glow.
Teacher's Tip: Fireflies (Jugnu) are rare examples of "living" natural light.
Exam Tip: The Sun is the most important answer to give here.

Question 4: List two artificial sources of light.
Answer: Electric bulb, torch, an oil lamp, fluorescent tube, candle.
Artificial sources are man-made inventions that convert other forms of energy, like electricity or chemical energy, into light. They allow us to see during the night or in dark indoor spaces.
Teacher's Tip: Anything you have to "turn on" or "light" with a match is artificial.
Exam Tip: "Candle" is a common artificial source that students sometimes mistake for natural.

Question 5: Differentiate between the luminous and non-luminous bodies. Give two examples of each.
Answer: Difference Between
Luminous: The bodies which have light of their own e.g. sun stars, bulb, candle, oil lamp, torch, a lantern.
Non-Luminous: The bodies-which do not have their own Iight.e.g. moon, chair, table. When light falls on them, they become visible.
Luminous bodies are the generators of light, while non-luminous bodies are the reflectors. Everything we see that isn't a light source is simply reflecting light from a luminous body.
Teacher's Tip: Luminous = Light Maker; Non-Luminous = Light Bouncer.
Exam Tip: Use a table format for differentiation questions to present your answer clearly.

Question 6: Is the moon a luminous object ?
Answer: Moon is not a luminous body, it is non-luminous body. It has no light of its own.
Even though it appears bright in the night sky, that brightness is just sunlight bouncing off the lunar surface. If the sun were to stop shining, the moon would immediately become dark.
Teacher's Tip: The moon is like a giant mirror in space!
Exam Tip: Always clarify that the moon "reflects" light from the sun.

Question 7: What do we call a body that shines on its own ?
Answer: The bodies that shines on its own or which themselves emit their own light are called the luminous bodies.
These bodies are active sources of light energy. They can be natural, like stars, or artificial, like a glowing LED.
Teacher's Tip: "Shine" means "Emit" in this context.
Exam Tip: The word "Luminous" comes from a root meaning "light."

Question 8: What do we call an electric bulb producing light ?
Answer: Luminous object.
Because it is generating light through its filament or gas, it is classified as a source. Specifically, it is an artificial luminous source.
Teacher's Tip: Any active light source is luminous.
Exam Tip: Be careful—a bulb that is switched "OFF" is non-luminous.

Question 9: What is a transparent medium ? Give two examples.
Answer: A medium which allows the passage of light through it easily, is called a transparent medium. Examples: glass, air, water etc.
Light rays travel straight through these materials without being scattered or blocked. This allows us to see images clearly through them.
Teacher's Tip: Transparent = See-through.
Exam Tip: Air is a transparent medium that is often forgotten because we can't see it!

Question 10: Explain the difference between a transparent, a translucent and an opaque medium. Give two examples of each.
Answer:
1. Transparent objects - Those objects through which light can pass easily are called transparent objects. e.g. Water, glass, air.
2. Translucent object- The object through which light can pass partially are called translucent object, e.g. tracing paper, waxed paper.
3. Opaque object- The objects which do not allow the light to pass through are called opaque objects, e.g. wood.
The amount of light allowed through determines the category. Transparency gives clear vision, translucency gives fuzzy vision, and opacity gives no vision at all.
Teacher's Tip: Use "Full," "Partial," and "None" to remember the levels of light passage.
Exam Tip: Provide two distinct examples for each to demonstrate a broad understanding.

Question 11: What do we call a substance through which we cannot see light ? Give an example of such a substance.
Answer: A substance through which we cannot see light is called an opaque medium. Examples: Wood, metals, butter paper and black paper etc.
These materials have molecules that are packed in a way that light energy is either absorbed or reflected away. This physical barrier stops light from penetrating to the other side.
Teacher's Tip: Opaque materials "hide" what's on the other side.
Exam Tip: Wood is the classic example for an opaque substance.

Question 12: What do we call a substance through which light passes ? Give an example of such a substance.
Answer: A substance through which light passes is called a transparent substance. Examples: glass, air, water etc.
Transparent substances have very little internal resistance to light. They allow light to maintain its straight-line path, preserving the image's detail.
Teacher's Tip: If light can "go through," it's transparent.
Exam Tip: Use "Clear Water" as an example to distinguish from murky water.

Question 13: Can a transparent medium form an image ? Explain your answer.
Answer: No, a transparent medium cannot form image. All the light that passes through a transparent medium completely pass through the substance. For the formation of image it is must that the light rays get reflected through the surface.
Images are formed when light is manipulated—either reflected by a mirror or refracted by a lens. A perfectly transparent sheet just lets light vanish through it, leaving nothing behind for your eye to "catch."
Teacher's Tip: Transparency is about letting light "pass," not about "catching" it.
Exam Tip: Mention reflection/refraction as the necessary processes for image formation.

Question 14: How can you obtain a point source of light ?
Answer: A point source of light is obtained either by placing a screen having a fine hole, in front of die luminous body or by placing the luminous body inside a box having a fine hole on one of its side.
By forcing light from a large source through a tiny hole, we simulate light coming from a single mathematical point. This is used in experiments to create very sharp shadows without penumbras.
Teacher's Tip: A point source is like a "Laser dot" on a wall.
Exam Tip: Explain that a point source creates a shadow with only an umbra.

Question 15: Define the terms : a ray of light and a beam of light.
Answer: The light travelling in any one direction in a straight line is called a ray of light. A group of light rays given out from a source is called a beam of light.
A ray is the simplest unit of light path, often represented by a straight line with an arrow. A beam is what we actually see in nature, like the light from a car headlight.
Teacher's Tip: Ray = One line; Beam = A bundle of lines.
Exam Tip: Always draw an arrow on your "Ray" diagrams to show the direction of light.

Question 16: What do you mean by ‘rectilinear propagation of light’ ?
Answer: Light travels in a straight line path. This is called the rectilinear propagation of light.
This is a fancy way of saying light doesn't like to turn corners. It is the most fundamental rule for how light behaves in a uniform medium.
Teacher's Tip: "Rectilinear" = "Straight Line."
Exam Tip: Use this term to explain why we cannot see things behind a wall.

Question 17: Describe an experiment to show that light travels in a straight line path.
Answer: Take three cardboards A, B and C each about 25 cm square. Take a pin and make a small hole in each cardboard at the same height. Suspend the cardboard pieces by separate threads vertically from a support such that each hole is at the same height, as shown. Pass a string through the holes and pull it taut. This makes the three holes in a straight line. Now take out the string. Place a lighted candle near one of the cardboards (say A). Look at the candle flame from the other side of the cardboard C. The candle flame is clearly seen. Now slightly displace one of the cardboards (say B) so that the holes no longer remain in a straight line. Again look at the candle flame from the other side of the cardboard C. You do not see the candle flame. The reason is that light travels in a straight line and now the holes in the cardboards A, B and C are not in a straight line. Conclusion: Light travels in a straight line path called the rectilinear propagation of light.
This experiment proves that light cannot bend to find its way through misaligned holes. It serves as a visual demonstration of light's rigid travel path.
Teacher's Tip: If you move one card, you "cut" the light beam.
Exam Tip: Be sure to describe the observation both before and after moving the cardboard.

Question 18: In which of the following two arrangements (a) and (b) shown in the diagram, you can see the light of the bulb ? Explain Your answer
Answer: We can see the light of the bulb in the arrangement (a). This is so because in arrangement ‘a’ the rod is straight and light travels in a straight line path. Whereas in arrangement ‘b’ the rod is bent. So the light cannot pass through it.
Because light rays travel rectilinearly, they collide with the walls of a bent tube and are absorbed. A straight tube provides a direct, unblocked path for the rays to reach your eye.
Teacher's Tip: Light is like a train that can't stay on a curved track!
Exam Tip: Mention "rectilinear propagation" in your explanation for more marks.

Question 19: Name a simple application of the rectilinear propagation of light
Answer: The simple applications of rectilinear propagation of light are pin hole camera, formation of shadows and elipses.
These phenomena rely entirely on the fact that light travels in straight lines and is blocked by opaque objects. Without this property, shadows would not be dark and pinhole cameras would only see blurry light.
Teacher's Tip: The Pinhole Camera is the most famous application of this rule.
Exam Tip: "Shadow formation" is the easiest example to remember and explain.

Question 20: What is a pin hole camera ? Draw a neat and labelled diagram to show the formation of image of a lighted candle by it.
Answer: The pin hole camera is a simple application of the rectilinear propagation of light.
It is a basic device without lenses that uses a tiny hole to project an inverted image of an object onto a screen. It demonstrates how light rays criss-cross through a single point.
Teacher's Tip: It's basically a box with a hole and a screen—very simple to build!
Exam Tip: When drawing, make sure the light rays from the top of the candle cross the hole to the bottom of the screen.

Question 21: Explain the formation of image of a luminous object in a pin hole camera with the aid of a neat diagram.
Answer: When a luminous object AB, such as a lighted candle, is placed in front of the pin hole, an inverted picture A’ B’ of the candle is obtained on the tracing paper. This picture A’ B’ is called the image. The image obtained is upside down (i. e. inverted). The reason is that the light travels in a straight line path. Hence light from the upper point A of the candle passes through the pin hole and strikes the tracing paper at A’. Similarly, light from the lower point B of the candle passes through the pin hole and strikes the tracing paper (or screen) at B’. Light from all the other points between A and B, on passing through the pin hole strikes the tracing paper in between A’ and B’. As a result, an inverted image of the candle is seen on the tracing paper Fig shows the simple ray diagram for die formation of image.
The small size of the hole ensures that only one ray from each point of the object can reach the screen. This mapping of points creates a faithful, though inverted, representation of the source.
Teacher's Tip: Think of the pinhole as a "Filter" that only lets one ray per point through.
Exam Tip: Use the labels A-B for the object and A'-B' for the image in your diagram.

Question 22: State two factors which affect the size of image formed in a pin hole camera.
Answer: Factors affecting the size of the image : The size of image depends on the following two factors:
1. The distance of screen (i.e. tracing paper) from the pin hole, and
2. The distance of object (i.e. candle) in front of the pin hole.
Moving the screen further back makes the image larger because the light rays have more room to spread out. Conversely, moving the object closer to the hole also increases the image size.
Teacher's Tip: More distance between hole and screen = Bigger image.
Exam Tip: Mention that the size of the image is "directly proportional" to the screen distance.

Question 23: Is the image obtained in a pin hole camera erect or inverted ? Give reason for your answer.
Answer: Image obtained in a pin hole camera is inverted. The reason is that the light travels in a straight line path. Hence light from the upper point of the candle passes through the pin hole and strikes the tracing paper in the lower point. Similarly light from the lower point of the candle passes through the pin hole and strikes the tracing paper at the upper point.
Since light rays cannot bend, they must cross paths at the tiny opening of the pinhole. This crossing causes the rays from the top to end up at the bottom and vice-versa, resulting in an upside-down image.
Teacher's Tip: "Inverted" is just the scientific way of saying "Upside Down."
Exam Tip: The "straight line path" is the most important part of the reason.

Question 24: How is the image affected in a pin hole camera when another fine hole is made near the first pin hole ?
Answer: If another pin hole is made near the first pin hole, two images are formed on the screen, one due to each of the two pin holes. If the holes are very close, the two images tend to overlap each other. As a result, a blurred image will be seen.
Each hole acts as its own projector, creating a separate image of the source. When these images are slightly offset, they blend together into a fuzzy mess rather than a sharp picture.
Teacher's Tip: Two holes = Two overlapping movies on the same screen.
Exam Tip: Use the word "overlap" to explain the blurring effect.

Question 25: State the effect on the image in a pin hole camera if 1. The hole is made bigger. 2. The luminous object is moved towards the pin hole. 3. The length of the pin hole camera is increased (le. the screen is moved away from the pin hole).
Answer:
1. If the hole is bigger than a pin hole, again a blurred image in seen. The reason is that a bigger hole is equivalent to a large number of pinholes. Each pin hole produces one image. These images overlap each other resulting in a blurred image.
2. If the object is moved towards the pin hole the size of the image increases.
3. When the length of the pin hole camera is increased. C is the screen is moved away from the pinhole, the size of image also increases.
Changing the geometry of the camera changes how light rays spread and overlap. A small hole and specific distances are required to maintain a balance between brightness and sharpness.
Teacher's Tip: A bigger hole makes the image "Brighter" but "Blurrier."
Exam Tip: Remember the formula: Size of image / Size of object = Distance of screen / Distance of object.

Question 26: What is a shadow ? Give a reason for its formation.
Answer: Shadow : When light falls on an opaque object, light is obstructed and a dark patch on a screen kept behind is called shadow. This is because light propagates in straight line. If distance between object and screen is less, the shadow will be (umbra) dark and smaller. If the distance is increased shadow will be dim and larger.
A shadow is simply the space where light is missing because an object got in the way. It serves as perfect proof that light cannot curve around objects to fill the darkness.
Teacher's Tip: A shadow is the "Light-Free Zone" created by an object.
Exam Tip: Mention "rectilinear propagation" as the reason for shadow formation.

Question 27: Draw a ray diagram to show the formation of shadow of an opaque object by a point source of light. How is the size of shadow affected if the screen is moved away from the object?
Answer: If we move the screen away from the object, the shadow increases in size.
As the distance increases, the cone of light blocked by the object has more room to expand outward. This makes the dark patch on the screen wider and taller.
Teacher's Tip: Think of a flashlight; the further you are from the wall, the bigger the light (and shadow) becomes.
Exam Tip: In a point source diagram, ensure there is only umbra and no penumbra.

Question 28: State two differences between an umbra and a penumbra.
Answer: Umbra: 1. It is the portion of shadow where no light reaches from the source of light due to the opaque object. 2. It is completely dark.
Penumbra: It is the portion of shadow where a portion of light from the source of light reaches the shadow even in the presence of the opaque object in between them. It is not completely dark, but is dim (or less bright).
The umbra is the central zone of total eclipse, while the penumbra is the outer fringe of partial light. Seeing a penumbra requires a large light source that light can "leak" around.
Teacher's Tip: Umbra = Total Dark; Penumbra = Part Dark.
Exam Tip: Use a table to compare these two parts of a shadow for clarity.

Question 29: Draw a ray diagram to show the formation of umbra alone.
Answer: Formation of umbra alone.
To get only an umbra, you must use a point source of light. In this case, the object blocks all available rays perfectly, leaving no partially-lit regions on the screen.
Teacher's Tip: A point source cannot produce a penumbra.
Exam Tip: Use a single dot to represent the "Source" in your diagram for this question.

Question 30: Draw a ray diagam to show the formation of umbra and penumbra both. Label the parts umbra and penumbra in your diagram.
Answer: If your move the screen away from the object, the shadow increases in size. Formation of umbra and penumbra both.
This requires an "extended" light source, like a long tube light or a large candle flame. The rays from the top and bottom of the flame create overlapping shadows, producing the two distinct zones.
Teacher's Tip: An extended source is like having many point sources lined up together.
Exam Tip: Clearly shade the umbra darker than the penumbra in your drawing.

Question 31: In each of the following diagrams, draw rays to form umbra and penumbra on the screen. (a) Point source (b) Extended source
Answer: (a)A’B’—umbra Umbra alone is obtained on the screen when the opaque object is illuminated by a point source of light.
Diagram (a) shows light rays diverging from a single dot, hitting the edges of the circular object. Diagram (b) would show light rays from two different points on the source crossing to create the penumbra.
Teacher's Tip: Remember: Point = One dark patch; Extended = Two shaded patches.
Exam Tip: Use a ruler to draw straight light rays so your diagram looks professional.

Question 32: State the conditions when only the penumbra of an opaque object is obtained on the screen.
Answer: If the size of source of light is bigger than the size of the opaque object, the size of umbra is very small. If the screen is moved away from the object, the umbra vanishes and only the penumbra remains.
When the light source is massive, light rays from the sides can "converge" behind a small object. At a great enough distance, these rays meet, effectively ending the total shadow (umbra).
Teacher's Tip: This is why tiny dust in the air doesn't cast a dark shadow on the floor!
Exam Tip: Mention "extended source" and "large distance" as the two key conditions.

Question 33: Why is it that the birds flying in the sky do not cast their shadow on the earth ?
Answer: We do not see the shadow of a bird flying high up in air because in their shadow, the umbra is absent and the penumbra is too large and too faint that it is not visible as the distance of screen (i.e. earth) is very large from the object (i.e. bird).
Because the bird is small compared to the sun, the dark part of its shadow (umbra) never reaches the ground. By the time the shadow reaches us, it's just a very light penumbra that is too spread out for our eyes to detect.
Teacher's Tip: The further the bird, the "fainter" its shadow becomes.
Exam Tip: Specify that the "umbra vanishes" before it reaches the ground.

Question 34: Why are shadows at noon shorter than in the morning or in the evening ?
Answer: At noon the sun is directly overhead. So, the sun rays fall vertically on the body. Hence the shadow is very short. In the morning and evenings, the sun rays fall in an inclined position. So, the shadows are long.
The angle of the light determines the length of the blocked area. Vertical light only casts a shadow directly beneath the object, while slanted light spreads the shadow out across the ground.
Teacher's Tip: Overhead light = Mini shadow; Slanted light = Mega shadow.
Exam Tip: Use the terms "vertical" and "inclined" (slanted) to describe the sun's rays.

Question 35: What is an eclipse ? Name the two types of eclipses.
Answer: Eclipses are the examples of formation of shadows in nature. There are two kinds of eclipses: 1. Lunar eclipse (the eclipse of the moon), and 2. Solar eclipse (the eclipse of the sun).
An eclipse is simply a giant natural shadow event involving the Sun, Earth, and Moon. They are predictable scientific events that demonstrate how light and celestial bodies interact.
Teacher's Tip: An eclipse is just a shadow in space!
Exam Tip: Be sure to know what body is being blocked in each type.

Question 36: When does a lunar eclipse take place ? Does it occur on every full moon’s night ?
Answer: A lunar eclipse occurs when the earth comes in between the sun and moon and casts its shadow on moon. On a full moon night, the moon rises in the east after sun sets in the west. On such a night, the sun and moon are on the opposite sides of the earth. The shadow of the earth falls on the surface of the moon therefore moonis not visible to us. This is lunar eclipse as shown in the figure.
No, it does not happen every month because the moon's orbit is tilted. Most full moons pass slightly above or below the Earth's shadow instead of through it.
Teacher's Tip: Earth blocks the Sun from reaching the Moon.
Exam Tip: Note that during a total lunar eclipse, the moon often turns a reddish color.

Question 37: Draw a diagram to show the formation of lunar eclipse.
Answer: [Diagram showing Sun, Earth, and Moon in order]
The diagram shows Earth in the center with a large cone-shaped shadow. The moon is seen moving through the umbra (total eclipse) and penumbra (partial eclipse) of Earth's shadow.
Teacher's Tip: Earth's shadow is much bigger than the moon, which is why lunar eclipses last a long time.
Exam Tip: Label the Umbra and Penumbra of Earth in your diagram.

Question 38: When does a solar eclipse take place ? Does it occur on even’ new moon’s day ?
Answer: Solar eclipse— On a certain moon’s day the moon, happens to come in between the sun and the earth. They come in a straight line. In such a situation, the moon being smaller in size casts its shadow only on a limited region on the earth. In these regions of the earth, the solar eclipse occurs.
Just like lunar eclipses, solar eclipses only happen when the alignment is perfect. Most new moons occur without an eclipse because the moon's shadow "misses" the Earth.
Teacher's Tip: Moon blocks the Sun from reaching us on Earth.
Exam Tip: A solar eclipse only affects a small part of the earth at one time.

Question 39: Draw a diagram to show the formation of solar eclipse.
Answer: [Diagram showing Sun, Moon, and Earth in order]
The diagram illustrates how the small moon casts a narrow shadow on the vast Earth. People standing in the tiny umbra see a total eclipse, while those in the wider penumbra see a partial one.
Teacher's Tip: The moon's shadow is like a small spotlight on the Earth.
Exam Tip: Ensure the moon is correctly placed between the sun and earth.

Question 40: What is annular solar eclipse ? Draw a labelled diagram to show its formation.
Answer: An annular solar eclipse occurs when only the tip of the umbra of the moon falls on the earth. From the point D, the sun will appear to be completely obstructed by the moon, only the outer rim of the sun, called corona, is then visible for a very short time which is known as the diamond ring. The formation of annular solar eclipse is shown below.
This happens when the moon is a bit further away from Earth than usual, so it appears slightly smaller in the sky. It cannot cover the entire sun, leaving a "ring of fire" visible around the moon's silhouette.
Teacher's Tip: Annular = "Ring-shaped."
Exam Tip: Use the term "Ring of Fire" to describe the visual appearance of an annular eclipse.

ADDITIONAL QUESTIONS

INTEXT QUESTIONS 1

Question 1: Name any four man made sources of light.
Answer: 1. An electric bulb, 2. Candle, 3. An oil lamp, 4. A torch.
These are all artificial inventions that humanity has developed to control darkness. They rely on burning fuels or using electricity to generate light rays.
Teacher's Tip: If you need batteries or a match, it's man-made!
Exam Tip: Make sure your examples are distinct, like a chemical flame vs. an electric filament.

Question 2: Why do objects in a room become visible even if sunlight does not enter it?
Answer: When light falls on an object, it scatters even if the sunlight does not enter a room. due to scattered light., the object become visible.
Light reflects off the ground, buildings, and dust in the air, bouncing into the room through windows. This "indirect" light provides enough illumination for us to see our surroundings.
Teacher's Tip: Scattering is like "splashing" light in every direction.
Exam Tip: Use the word "scattering" or "diffuse reflection" to explain this.

Question 3: Does the flame of a gas stove emit light?
Answer: Any burning thing emit light. The intensity of light may not be same. A flame of a gas stove emit light.
Even though gas stove flames are primarily used for heat, the chemical reaction of burning gas produces a blue glow. This emission of photons makes the flame a luminous source.
Teacher's Tip: If you can see it in a pitch-black room, it's emitting light.
Exam Tip: The brightness or "intensity" doesn't change the fact that it's a light source.

Question 4: Name few living things which emit light.
Answer: Jugnu and firefly.
These insects use a process called bioluminescence to create light in their bodies. They use these flashes to communicate with other members of their species.
Teacher's Tip: "Bioluminescence" is the scientific word for living light.
Exam Tip: Jugnu is the common Hindi name for a firefly.

INTEXT QUESTIONS 2

Question 1: From among the following objects, list out the opaque, translucent and transparent bodies: A glass strip, an ordinary paper, greased paper, tracing paper, polythene sheet, wooden piece, steel plate, glycerine, kerosene oil, mustard oil, grounded glass, stone, book, water and milk.
Answer:
Opaque: wooden piece, steel plate, stone, book, milk.
Transparent: A glass strip, kerosene oil, water.
Translucent: greased paper, tracing paper, polythene sheet, glycerine, mustard oil, grounded glass.
This categorization depends on how the internal structure of the substance interacts with incoming light rays. Pure liquids are often transparent, while oils and textured papers tend to scatter light partially.
Teacher's Tip: Milk is opaque because the fat particles block all the light!
Exam Tip: Be careful with "grounded glass"—it's the same as "frosted glass" and is translucent.

INTEXT QUESTIONS 3

Question 1: Write the names of some transparent objects.
Answer: Glass, water, air etc.
These materials allow light to pass through in a straight line with minimal absorption. Because they don't scatter light, we can see perfect images through them.
Teacher's Tip: Air is the transparent medium we use every single second!
Exam Tip: Always list "Glass" and "Water" as your primary examples.

Question 2: What happens in a solar eclipse ?
Answer: A solar eclipse occurs when the moon comes in between the Sun and the Earth and casts its shadow on the earth. In such a case, the moon being smaller in size casts its shadow only on a limited region on the earth.
During this time, the day can briefly turn as dark as night for people inside the moon's umbra. It is a striking visual reminder of the constant motion of our solar system.
Teacher's Tip: The Sun "disappears" because the Moon is in the way.
Exam Tip: Mention that it only affects a "limited region" of the Earth at any one time.

Question 3: What do you understand by candle power ?
Answer: Candle power — To measure the brightness of light in comparison with the brightness of a candle. The unit of candle power is used.
Before modern units like lumens were common, scientists used the light of a standard-sized wax candle as a reference. It tells us how much "glowing power" a source has relative to that baseline.
Teacher's Tip: It's an old-fashioned way of saying "How many candles is this bulb worth?"
Exam Tip: Note that "Candela" is the modern S.I. unit derived from this concept.

Question 4: Write the names of some cold light emitting objects.
Answer: Jugnu, firefly.
"Cold light" means light produced without significant heat. Most man-made sources like bulbs get very hot, but bioluminescent creatures produce light efficiently without burning themselves.
Teacher's Tip: Cold light is much more energy-efficient than hot light.
Exam Tip: LEDs are also considered a form of "cold light" in modern technology.

EXERCISE

A. Tick the correct answer

1. Light causes
1. Sensation of heat 2. Sensation of vision 3. Sensation of sound 4. None of these
Answer: 2. Sensation of vision
While sunlight carries heat (infrared), the primary function of visible light as we define it is to trigger our sense of sight. Vision is the brain's interpretation of light waves hitting our eyes.
Teacher's Tip: Our eyes are "Light Sensors."
Exam Tip: Don't get confused by the heat of the sun; the scientific definition of light links to vision.

2. Light is a form of
1. Work 2. Energy 3. Power 4. None of these
Answer: 2. Energy
Light can do work, such as powering solar cells, which proves it is a form of energy. It travels across the vacuum of space in the form of electromagnetic waves.
Teacher's Tip: Energy is the "Fuel," and light is a very fast type of fuel.
Exam Tip: Energy is the broad category for all phenomena like light, sound, and heat.

3. The natural source of light is
1. Sun 2. Electric lamp 3. Candle flame 4. Kerosene lamp
Answer: 1. Sun
The Sun is a natural nuclear reactor that has been shining for billions of years without human help. All the other options are items created by people.
Teacher's Tip: If nature made it, it's a natural source.
Exam Tip: The Sun is the ultimate source of almost all energy on Earth.

4. A tubelight is a
1. Hot source 2. Cold source 3. Incandescent source 4. None of these
Answer: 2. Cold source
Fluorescent tubelights do not rely on a hot wire (filament) to glow; they use gases that emit light with very little heat. This makes them much cooler to the touch than old-style bulbs.
Teacher's Tip: "Cold" just means it doesn't get as hot as a fire or a regular bulb.
Exam Tip: Modern LEDs are also examples of cold sources.

5. Glass is
1. Opaque 2. Luminous 3. Transparent 4. None of these
Answer: 3. Transparent
Glass is designed to allow light to pass through it cleanly so we can see objects on the other side. This property is why it is used for windows and cameras.
Teacher's Tip: If you can see through it, it's transparent.
Exam Tip: Frosted glass is a rare exception (it's translucent).

6. Substance which don’t allow light to pass through them,
1. Transparent substance 2. Opaque substance 3. Translucent substances 4. Non-luminous substance
Answer: 2. Opaque substance
Opaque substances like rocks or metal create a physical barrier that stops light rays. This causes a shadow to form behind the object.
Teacher's Tip: Opaque = Blocks light.
Exam Tip: Note that non-luminous just means it doesn't *make* light, but it could still be transparent (like water).

7. The image formed in a pinhole camera is
1. inverted 2. upright 3. always enlarged 4. always diminished
Answer: 1. inverted
Because light rays travel in straight lines and cross over at the pinhole, the top of the object appears at the bottom of the screen. This is a classic demonstration of rectilinear propagation.
Teacher's Tip: Inverted = Upside Down.
Exam Tip: The size can be enlarged OR diminished depending on distances, but it's *always* inverted.

8. Shadow forms on the
1. Opposite side of the object 2. Same side of the object 3. On the same plane of the object 4. None of these
Answer: 1. Opposite side of the object
Since the object blocks light from the source, the darkness must appear on the side away from the light. This is why your shadow is always behind you when you walk toward a lamp.
Teacher's Tip: Light and shadows are like two people on opposite ends of a seesaw.
Exam Tip: A shadow always requires a light source, an object, and a screen on the opposite side.

9. When the moon comes in between the sun and the earth, and all three are in a straight line, then it is
1. Solar eclipse 2. Lunar eclipse 3. Either solar eclipse or lunar eclipse 4. No eclipse
Answer: 1. Solar eclipse
In this position, the moon blocks the sun's rays from reaching parts of the earth. We see the moon's silhouette moving across the disk of the sun.
Teacher's Tip: Solar = Sun. The Sun gets "eclipsed" or blocked.
Exam Tip: Remember the sequence: Sun - Moon - Earth.

10. Umbra is a region of
1. Partial darkness 2. Partial brightness 3. Complete brightness 4. Complete darkness
Answer: 4. Complete darkness
The umbra is the darkest part of the shadow where the light source is totally hidden. No direct rays from any part of the source reach this area.
Teacher's Tip: Umbra is the "darkest dark."
Exam Tip: Only umbra exists if the light source is a single "point."

B. Fill in the blanks

1. The Sun and moon are the natural sources of light.
Answer: Sun, moon.
Note: Scientifically, the moon is a reflector, but it is often grouped with natural celestial sources in introductory lessons. The sun is the primary natural emitter.
Teacher's Tip: Use "Sun" for emission and "Moon" for reflection.
Exam Tip: If you need a "source" that makes its own light, the Sun is a better answer than the Moon.

2. There are natural and man-made sources of light.
Answer: natural, man-made.
This classification divides light sources based on their origin. Natural sources come from the universe, while man-made ones are human technology.
Teacher's Tip: Man-made sources are also called "Artificial."
Exam Tip: Be prepared to give examples for both categories.

3. The brightness of light is measured in candle power.
Answer: candle power.
This term relates the intensity of a light source to a standard wax candle. Modern scientists use the unit "candela" for this purpose.
Teacher's Tip: Think of it as a measurement of "Light Strength."
Exam Tip: Learn the definition of "lumen" as a related concept for brightness.

4. Shadows are formed when objects blocks light.
Answer: objects.
Specifically, these must be opaque or translucent objects that interfere with the path of light. A perfectly transparent object will not cast a noticeable shadow.
Teacher's Tip: No block = No shadow.
Exam Tip: Mention "opaque objects" for the darkest shadows.

5. Eclipses are the formation of shadows in nature.
Answer: shadows.
An eclipse is simply a large-scale version of the shadow an object casts on a wall. It involves planets and moons instead of toys and flashlights.
Teacher's Tip: The Earth and Moon are the "Opaque Objects" in an eclipse.
Exam Tip: Use this to explain why eclipses follow the rules of rectilinear propagation.

6. A lunar eclipse is formed on full moon day.
Answer: full moon.
During a full moon, the Earth is between the Sun and the Moon. If the alignment is straight, the Earth's shadow covers the moon.
Teacher's Tip: Lunar eclipse = Earth's shadow on the Moon.
Exam Tip: Solar eclipses happen on New Moon days; Lunar on Full Moon nights.

7. A shadow is formed when the path of light is obstructed by an object.
Answer: object.
The object stops the light from reaching the screen, creating a dark patch. This demonstrates that light cannot "dodge" or curve around obstacles.
Teacher's Tip: Obstruction is the act of blocking.
Exam Tip: This is the core reason why light's path is described as "rectilinear."

8. Light consists of seven colours.
Answer: seven.
When white light passes through a prism, it splits into red, orange, yellow, green, blue, indigo, and violet. This combination of colors is called the spectrum.
Teacher's Tip: Use the name "VIBGYOR" to remember them in order.
Exam Tip: A rainbow is a natural example of white light splitting into seven colors.

9. Pinhole camera is based on the principle propagation of light.
Answer: rectilinear.
Because light moves only in straight lines, a small hole can organize light rays from an object into an image. If light curved, it would just fill the camera with blurry light.
Teacher's Tip: Rectilinear propagation is the "Golden Rule" for this device.
Exam Tip: This principle also explains why the image is inverted.

10. A solar eclipse occurs only on a New Moons Day.
Answer: New Moons Day.
On this day, the moon is positioned between the earth and the sun. This is the only phase where the moon's shadow can fall onto our planet.
Teacher's Tip: Solar = Sun = New Moon.
Exam Tip: Remember it doesn't happen on *every* new moon day due to orbital tilt.

11. Speed of light in vacuum is 300000 km/sec
Answer: 300000 km/sec.
Light is the fastest thing in the universe. At this speed, it can travel around the entire Earth seven times in just one second.
Teacher's Tip: That is roughly 3 \times 10⁸ meters every single second!
Exam Tip: Be careful with units; sometimes questions use m/s instead of km/s.

C. Match the following

1. Transparent object --- (a) Football
2. Eclipses --- (b) Glass
3. Translucent object --- (c) Fluorescent tube
4. Luminous object --- (d) Tracing paper
5. Non-luminous object --- (e) Shadows
Answer:
1. Transparent object - (b) Glass
2. Eclipses - (e) Shadows
3. Translucent object - (d) Tracing paper
4. Luminous object - (c) Fluorescent tube
5. Non-luminous object - (a) Football
This matching activity helps you identify how different materials interact with light rays. It also connects natural phenomena like eclipses to the basic scientific concept of shadow formation.
Teacher's Tip: Remember the "Three Ts": Transparent, Translucent, and Tracing paper all start with 'T', but only one is an example of the other!
Exam Tip: For scoring full marks, draw straight lines between the columns or rewrite the pairs clearly to make it easy for the examiner to read.

D. Write true or false

1. The moon has its own light.
Answer: False
It is a non-luminous celestial body. The light we see from the moon is simply sunlight reflected from its surface toward Earth.
Teacher's Tip: No sun = No moonlight.
Exam Tip: Reflection is the process that makes the moon visible.

2. The earth has its own light.
Answer: False
The Earth is an opaque, non-luminous planet. It reflects the sun's light, which is how astronauts can see the "Blue Marble" from space.
Teacher's Tip: Planets are non-luminous; stars are luminous.
Exam Tip: Earth is visible to others only by reflection.

3. The stars have no light.
Answer: False
Stars are massive luminous spheres of gas, just like our sun. They produce immense amounts of light and heat through nuclear reactions.
Teacher's Tip: The Sun is just the closest star to us!
Exam Tip: Stars are prime examples of natural luminous sources.

4. Opaque objects do not allow light to pass through them.
Answer: True
Materials like wood, stone, and metal block light completely. This creates the dark region behind them known as a shadow.
Teacher's Tip: If you can't see through it, it's opaque.
Exam Tip: Opacity is the condition for shadow formation.

5. Image formed in a pinhole camera is erect.
Answer: False
The image is always inverted (upside down). This is due to the straight-line travel of light as rays from the top and bottom of the object cross through the pinhole.
Teacher's Tip: Erect means "Right-side up," which is the opposite of inverted.
Exam Tip: "Inverted" is the most characteristic property of a pinhole camera image.

E. Answer the following

Question 1: What is light ?
Answer: Light is a form of Energy i.e. The external physical cause that affects our eye to produce the sensation of vision.
It consists of electromagnetic waves that travel through vacuum and air. When these waves hit the retina of our eyes, they create the signals our brain interprets as "seeing."
Teacher's Tip: Light = Energy + Vision.
Exam Tip: Use "form of energy" to start your definition for higher marks.

Question 2: List four natural sources of light.
Answer: Sun, stars, jugnu, firefly.
Natural sources provide light without any human technology. They can be massive like stars or tiny biological beings like fireflies in a dark forest.
Teacher's Tip: The Sun is the biggest and brightest of them all.
Exam Tip: Listing "stars" covers billions of natural sources at once!

Question 3: Write down five man-made sources of light.
Answer: Electric bulb, torch, an oil lamp, fluorescent tube, candle.
Humanity has engineered these to ensure we have visibility after sunset. They transform energy from chemicals (candles/oil) or electricity (bulbs) into light.
Teacher's Tip: Artificial sources are for our convenience.
Exam Tip: "LED lamp" is another great modern example to include.

Question 4: What is meant by luminous intensity ? What is one lumen ?
Answer: Luminous Intensity— The measurement of brightness of light is called luminous intensity. One lumen is considered to 1 / 1256 th of a candle power.
Luminous intensity tells us how "powerful" a light source is in a particular direction. The lumen is the unit used to measure the total amount of visible light emitted by a source.
Teacher's Tip: Bright bulbs have high "Lumen" ratings on their boxes.
Exam Tip: Memorize the fraction 1/1256 for the definition of one lumen.

Question 5: Distinguish between transparent, translucent and opaque objects.
Answer:
1. Transparent objects - Those objects through which light can pass easily are called transparent objects, e.g. Water, glass, air.
2. Translucent object- The object through which light can pass partially are called translucent object, e.g. tracing paper, waxed paper.
3. Opaque object- The objects which do not allow the light to pass through are called opaque objects, e.g. wood.
The main difference is how much light the material "lets through." Transparent allows all, translucent allows some (and scatters it), and opaque allows none.
Teacher's Tip: Transparent is like a clear window; Translucent is like a foggy window.
Exam Tip: Use a table to organize these three categories and their examples.

Question 6: List two transparent objects.
Answer: Water, glass.
These materials are crucial for optical tools like lenses and glasses. Because they don't block light, we can see exactly what is on the other side.
Teacher's Tip: Pure air is also a great example of transparency.
Exam Tip: Mention "Clean Water" for precision.

Question 7: Name some sources of light that are not hot.
Answer: Moon, jugnu.
While the moon reflects light, the Jugnu (firefly) generates "cold light" chemically. These sources provide illumination without creating a high temperature.
Teacher's Tip: Cold light is very efficient because it doesn't waste energy as heat.
Exam Tip: Jugnu is the best biological example of cold light.

Question 8: How is a shadow formed ? Write the conditions required for a shadow to form.
Answer: Shadow: When light falls on an opaque object, light is obstructed and a dark patch on a screen kept behind is called shadow. This is because light propagates in straight line. If distance between object and screen is less, the shadow will be (umbra) dark and smaller. Conditions Required for a shadow to form are:
1. There should be a source of light.
2. An opaque object.
3. A screen.
4. The opaque object is placed between the source of light and the screen.
Shadows are the result of light being "cut off" by an obstacle. Since light cannot bend to go around the obstacle, the area directly behind it remains dark.
Teacher's Tip: You need a source, a blocker, and a surface to see the shadow.
Exam Tip: List the four conditions as a numbered list for full points.

Question 9: Differentiate between luminous and non-luminous objects.
Answer: Difference Between
Luminous: The bodies which have light of their own e.g. sun stars, bulb, candle, oil lamp, torch, a lantern.
Non-Luminous: The bodies-which do not have their own light. E.g. moon, chair, table. When light falls on them, they become visible.
A luminous object is a "Light Giver," whereas a non-luminous object is a "Light Receiver." Without luminous objects, non-luminous ones would remain completely invisible to us.
Teacher's Tip: Stars glow; planets reflect.
Exam Tip: Remember that non-luminous objects are seen only by "reflection."

Question 10: How can you see the solar eclipse safely ?
Answer: To see the Solar Eclipse Safely: Take a cardboard or X-Ray film and make a small hole in it. Place it in front of a wall. Through the hole, the image of the Sun will be formed on the wall. This image we can see with a eye. We are not looking at the Sun directly, so there will be no harm to the eyes.
This method uses the pinhole camera principle to project a safe image. Looking directly at the sun during an eclipse can cause permanent damage to your retina due to intense rays.
Teacher's Tip: Never look at the Sun with naked eyes, even during an eclipse!
Exam Tip: Explain that "indirect viewing" is the safest way to witness a solar eclipse.