Frank Brothers Solutions for ICSE Class 9 Physics Chapter 6.1 Light Reflection Of Light

Question 1. Define light and state its primary function.
Answer: Light may be defined as the radiant energy which produces in us the sensation of sight. Light itself is invisible but makes other objects visible.
In simple words: Light is a form of energy that we cannot see directly, but it bounces off things and enters our eyes so that we can see the world.

📝 Teacher's Note: Explain that light is like a messenger. It carries information from an object to our eyes. Without it, even if our eyes are perfect, we are in total darkness.

🎯 Exam Tip: Always mention that light is a form of "radiant energy" and responsible for the "sensation of sight" to get full marks.

Question 2. Is light a form of energy?
Answer: Yes, light is a form of energy that produces the sensation of vision in our eyes.
In simple words: Yes, just like heat or electricity, light is energy that specifically helps us see things.

📝 Teacher's Note: You can give examples of light energy being converted, like solar panels turning light into electricity, to prove it is a form of energy.

🎯 Exam Tip: Use the term "sensation of vision" to describe the effect of light energy on human eyes.

Question 3. What is the velocity of light in a vacuum?
Answer: The velocity of light in vacuum is \( 3 \times 10^{8} \text{ m/s} \).
In simple words: Light is the fastest thing in the universe, traveling at 300 million meters every single second!

📝 Teacher's Note: Help students visualize this speed: Light can travel around the Earth about 7.5 times in just one second.

🎯 Exam Tip: Remember the units (\( \text{m/s} \)) and the power of 10 (\( 10^{8} \)). Writing \( 3 \times 108 \) is a common mistake.

Question 4. Name the two main sources of light with examples.
Answer: Two sources of light are

• Natural, for e.g. Sun
• Artificial, for e.g. Light bulb.

In simple words: Light comes from nature (like the sun) or from things humans made (like lamps and torches).

📝 Teacher's Note: Ask students to categorize other sources like fireflies (natural) and candles (artificial) to check their understanding.

🎯 Exam Tip: When giving examples, always provide at least one natural and one artificial source for a complete answer.

Question 5. List four characteristics of light.
Answer: Four characteristics of light are :-

• Light waves can travel through vacuum
• Light waves are transverse waves
• Wavelength of light waves is short so that their length is measured in centi-microns.
• The velocity of light in vacuum is \( 3 \times 10^{8} \text{ m/s} \).

In simple words: Light doesn't need air to travel, moves in a specific way called transverse waves, is very tiny in size, and moves extremely fast.

📝 Teacher's Note: Contrast light with sound: sound needs a medium (air/water) to travel, but light can travel through empty space (vacuum).

🎯 Exam Tip: The fact that light travels through a vacuum is a high-priority point for examiners.

Question 6. Give two examples of luminous bodies.
Answer: The Sun and the stars are the two luminous bodies.
In simple words: Luminous bodies are things that create and give out their own light, like a glowing star.

📝 Teacher's Note: Emphasize that "luminous" means the object is the source of the light, not just reflecting it.

🎯 Exam Tip: Do not use the moon as an example here; it is non-luminous.

Question 7. Give two examples of non-luminous bodies.
Answer: Two non-luminous bodies are moon, chair.
In simple words: Non-luminous bodies are objects that don't make their own light. We see them because light from somewhere else bounces off them.

📝 Teacher's Note: Many students think the moon is luminous. Clarify that it only shines because it reflects sunlight.

🎯 Exam Tip: Any everyday object like a book, table, or wall is a valid example of a non-luminous body.

Question 8. Distinguish between convergent and divergent beams of light.
Answer:

In simple words: Convergent rays come together at one spot (like a magnifying glass focusing sunbeams), while divergent rays spread out (like light from a torch).

📝 Teacher's Note: Use your fingers to demonstrate: pull them together for convergent and spread them apart for divergent.

🎯 Exam Tip: When distinguishing between the two, mention the type of lens (convex for convergent, concave for divergent) to show deeper knowledge.

Question 9. Define a ray of light.
Answer: A ray of light is the path along which light travels.
In simple words: A ray is like a single invisible line that light follows as it moves from one place to another.

📝 Teacher's Note: In diagrams, we represent a ray as a straight line with an arrow to show the direction of travel.

🎯 Exam Tip: A ray is a theoretical concept used to simplify how we draw light behavior in physics.

Question 10. State three distinctions between light and sound waves.
Answer: Three distinctions between light and sound waves are

• Light waves can travel through vacuum while sound waves cannot.
• Light waves are transverse waves while sound waves are longitudinal waves.
• The velocity of light in air is \( 3 \times 10^{8} \text{ m/s} \) while the speed of sound in air is just about \( 330 \text{ m/s} \).

In simple words: Light is much faster than sound, moves through empty space where sound cannot, and "vibrates" in a different direction.

📝 Teacher's Note: Use the example of lightning and thunder: you see the light instantly because it's fast, but you hear the sound later because it's slow.

🎯 Exam Tip: The difference in speed (\( 3 \times 10^{8} \) vs \( 330 \)) is the most common comparison used in exams.

Question 11. Identify the following: (i) A single path of light, (ii) Rays spreading out, (iii) Rays coming together, (iv) Rays staying the same distance apart.
Answer:
(i) A ray of light
(ii) Divergent beam of light
(iii) Convergent beam of light
(iv) Parallel beam of light
In simple words: These are the names we give to light depending on how the rays are moving—alone, spreading, closing in, or straight.

📝 Teacher's Note: Draw these four types on the board side-by-side to help students memorize the visual patterns.

🎯 Exam Tip: "Convergent" and "Divergent" are often confused; remember "Converge" means "come together."

Question 12. What is a medium in the context of light?
Answer: The substance through which light is made to pass is called medium.
In simple words: A medium is just the "stuff" light is traveling through, like air, water, or glass.

📝 Teacher's Note: Different mediums can slow down light or change its direction (refraction).

🎯 Exam Tip: "Transparent," "translucent," and "opaque" are the three types of mediums you should be familiar with.

Question 13. Define rectilinear propagation of light.
Answer: Rectilinear propagation of light is that the light travels along a straight line.
In simple words: Light doesn't curve or turn corners on its own; it always moves in a straight line as long as nothing blocks it.

📝 Teacher's Note: This is why shadows are sharp. If light could curve around objects, there would be no dark shadows.

🎯 Exam Tip: "Straight line" is the key phrase needed for this definition.

Question 14. Is glass an opaque medium?
Answer: No, glass is a transparent medium.
In simple words: No, glass lets almost all light pass through it, which is why we can see clearly through windows.

📝 Teacher's Note: Explain that "transparent" means most light passes through, "translucent" means some light passes, and "opaque" means no light passes.

🎯 Exam Tip: Transparent objects like clear water, air, and glass are the most common examples in this category.

Question 15. Are metals transparent by nature?
Answer: No, no metal is transparent by nature.
In simple words: Metals are opaque, meaning they block all light from passing through them.

📝 Teacher's Note: Metals are usually excellent reflectors of light, which is why we use them to make mirrors.

🎯 Exam Tip: Metals are the primary example of opaque materials in physics.

Question 16. State the differences between a ray of light and a beam of light.
Answer:

In simple words: A ray is like a single thread of light, while a beam is like a whole bunch of threads bundled together.

📝 Teacher's Note: A beam is technically a collection of rays. Think of a beam as a group of people and a ray as one person.

🎯 Exam Tip: The simplest distinction is that a beam is a "collection of rays."

Question 17. Identify the translucent medium among the following: clear water, air, waxed paper, iron plate.
Answer: Waxed paper is the translucent medium among the given substances.
In simple words: Waxed paper lets some light through, but it's blurry, so you can't see through it clearly.

📝 Teacher's Note: Other common translucent materials include frosted glass, tracing paper, and some plastics.

🎯 Exam Tip: Translucent materials "scatter" light, which is why we can't see distinct images through them.

Question 18. Illustrate parallel and divergent beams of light.
Answer: Parallel beam of light is the beam in which rays of light travel parallel to each other. Divergent beam of light is the beam of light in which rays of light diverge from each other as the beam progresses.
In simple words: Parallel rays stay like train tracks, never touching. Divergent rays start from one point and spread out like a fan.

📝 Teacher's Note: A searchlight produces a parallel beam, while a small LED bulb produces a divergent beam.

🎯 Exam Tip: Always use arrows in your diagrams to show the direction of the light rays.

Question 19. State two observations that prove light travels in a straight line.
Answer: Two observations that proves that light travels in a straight line are :-

• Sunlight coming through a hole in a dark room, we can easily see that light travels in a straight line
• Light coming from a laser light, used for presentation, can also be seen to travel in straight line.

In simple words: You can see this when sunbeams shine through a window in a dusty room or when you use a laser pointer; the path is always a straight beam.

📝 Teacher's Note: You can also demonstrate this with three cardboard sheets with holes: only when the holes are perfectly aligned in a straight line can you see the candle flame through them.

🎯 Exam Tip: Using the "laser light" example is a modern and effective observation to cite in exams.

Question 20. Define reflection of light.
Answer: When rays of light fall on a surface, they are turned back into the same medium in accordance with some definite laws. This process is known as reflection.
In simple words: Reflection is when light hits a surface and bounces back, like a ball bouncing off a wall.

📝 Teacher's Note: Reflection is the reason we can see non-luminous objects. They reflect the light from a source into our eyes.

🎯 Exam Tip: Include the phrase "turned back into the same medium" to distinguish reflection from refraction.

Question 21. What causes a regular reflection?
Answer: A smooth and polished flat surface is the cause of regular reflection.
In simple words: When a surface is perfectly smooth, like a mirror or still water, it bounces all light rays in the same direction, creating a clear image.

📝 Teacher's Note: Contrast this with irregular reflection, which happens on rough surfaces like paper or a wall, where light bounces in many directions.

🎯 Exam Tip: Use the words "smooth" and "polished" to describe the surface for regular reflection.

Question 22. State the laws of reflection.
Answer: Reflection obeys following two laws

• The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.
• The angle of incidence and the angle of reflection are always equal.

In simple words: 1. The rays and the middle line all sit on the same flat surface. 2. The angle the light comes in at is exactly the same as the angle it bounces out at.

📝 Teacher's Note: If light hits a mirror at \( 30^\circ \), it *must* bounce off at exactly \( 30^\circ \). This is a fundamental rule of optics.

🎯 Exam Tip: Use a neat diagram to illustrate these laws; it's the best way to earn full marks.

Question 23. What should be the height of a plane mirror to see a full image? If a man is 1.6 m tall, what mirror size does he need?
Answer: The height of plane mirror should be half of the size of the object to get a full image of the object. So for a man of height 1.6 m tall should use a 0.8m tall plane mirror.
In simple words: To see your whole self in a mirror, the mirror only needs to be half as tall as you are.

📝 Teacher's Note: This is a cool geometry trick. Even if you stand further away, the part of the mirror you use for a full-body view stays the same.

🎯 Exam Tip: Remember the "half-size" rule for mirror calculations in exams.

Question 24. What is the straight-line travel of light called?
Answer: Rectilinear propagation of light is that the light travels along a straight line.
In simple words: This is just the scientific name for the fact that light travels in straight lines.

📝 Teacher's Note: This is a repetition to reinforce the terminology. "Rectilinear" means straight lines.

🎯 Exam Tip: Spelling "Rectilinear" correctly is important!

Question 25. Define lateral inversion.
Answer: Lateral inversion is the reversal of image experienced in a plane mirror. The image is of the same size and equidistant from the object but the left and right sides are transposed.
In simple words: This is why when you raise your right hand in front of a mirror, your reflection raises its left hand. Everything is flipped sideways.

📝 Teacher's Note: This is why the word "AMBULANCE" is written backward on the front of the vehicle—so drivers can read it correctly in their mirrors.

🎯 Exam Tip: Mention that the image is the same size and distance, but with "swapped sides."

Question 26. On what principle is the formation of images based?
Answer: Formation of image is the phenomenon based on laws of reflection.
In simple words: Images are created because light bounces off things in a predictable way.

📝 Teacher's Note: Reflection at a smooth surface redirects light rays to converge or appear to diverge from a specific point, which we perceive as an image.

🎯 Exam Tip: The "laws of reflection" are the fundamental building blocks for image formation.

Question 27. State the principle employed in a periscope.
Answer: The principle employed in a periscope is successive reflections from two plane mirrors.
In simple words: A periscope uses two mirrors at angles to bounce light from above down into your eyes, letting you see around corners or over walls.

📝 Teacher's Note: The mirrors are usually placed at a \( 45^\circ \) angle. Submarines use periscopes to see what's happening on the ocean surface.

🎯 Exam Tip: "Successive reflections" is the precise term to use for periscope operation.

Question 28. What is the point of incidence?
Answer: The point at which the light is incident on the reflecting surface is called the point of incidence.
In simple words: This is the exact spot where a ray of light hits the mirror.

📝 Teacher's Note: We always draw the "Normal" (the perpendicular line) starting from this exact point.

🎯 Exam Tip: In ray diagrams, ensure the incident ray, reflected ray, and normal all meet at the Point of Incidence.

Question 29. Define a mirror.
Answer: Any smooth, highly polished reflecting surface is called mirror.
In simple words: A mirror is anything so smooth and shiny that it reflects light perfectly to show an image.

📝 Teacher's Note: Most modern mirrors are made of glass with a thin coating of silver or aluminum on the back.

🎯 Exam Tip: The definition must include "smooth" and "polished" to be accurate.

Question 30. What is a plane mirror?
Answer: A smooth, highly polished plane surface is called plane mirror.
In simple words: A plane mirror is just a mirror that is perfectly flat, like the one in your bathroom.

📝 Teacher's Note: "Plane" in math and physics means "flat." It produces an image that is same-size and right-side up.

🎯 Exam Tip: Contrast this with spherical mirrors (convex/concave) which are curved.

Question 31. Define the angle of incidence.
Answer: The angle between the incident ray and the normal to the surface at the point of incidence is called the angle of incidence.
In simple words: It's the gap between the incoming light ray and the imaginary line sticking straight up from the mirror.

📝 Teacher's Note: A common mistake is measuring the angle between the ray and the *mirror*. Always measure from the *normal*.

🎯 Exam Tip: In diagrams, the angle of incidence is usually labeled as \( i \).

Question 32. If the angle of incidence is \( 20^\circ \), find the angle of reflection and the total angle between the incident and reflected rays.
Answer: Angle of reflection to the normal = angle of incidence to the normal = \( 20^\circ \)
So the angle between incident ray and reflected ray = Angle of incidence + Angle of reflection = \( 20 + 20 = 40^\circ \).
In simple words: Since the law says angles must be equal, reflection is \( 20^\circ \). The whole corner from one ray to the other is \( 20 + 20 = 40^\circ \).

📝 Teacher's Note: Use this to reinforce that the Law of Reflection (\( i = r \)) always applies on smooth surfaces.

🎯 Exam Tip: The "total angle" is always twice the angle of incidence.

Question 33. If a ray makes an angle of \( 35^\circ \) with the mirror surface, find the angles of incidence, reflection, and the total angle turned.
Answer: Given, angle between incident ray and mirror \( = 35^\circ \)

• Angle of incidence = angle of mirror to the normal - angle between incident ray and mirror
  \( = 90 - 35 = 55^\circ \)
• Angle of reflection = angle of incidence \( = 55^\circ \)
• Total angle turned = angle of incidence + angle of reflection
  \( = 55 + 55 = 110^\circ \)
• The angle between incident ray and reflected ray = Angle of incidence + Angle of reflection
  \( = 55 + 55 = 110^\circ \)

In simple words: First, find the angle to the vertical (\( 90 - 35 = 55 \)). Then use the law of reflection (\( 55 \)). Adding them together gives the total gap between rays.

📝 Teacher's Note: This is a very common trick question. Remind students that \( i \) and \( r \) are measured from the *normal*, not the mirror.

🎯 Exam Tip: Always subtract the "glancing angle" from \( 90^\circ \) to find the actual angle of incidence.

Question 34. A boy is 3 m from a mirror. (i) What is the distance between him and his image? (ii) If he moves to 4 m, what is the new distance?
Answer: Given, distance of boy from the mirror \( = 3\text{ m} \)

• Distance of image from mirror = distance of boy from the mirror \( = 3\text{ m} \)
  Distance between boy and his image = distance of boy from the mirror + distance of image from mirror \( = 3 + 3 = 6\text{ m} \)
• Now, distance of boy from the mirror \( = 4\text{ m} \)
  Distance of image from mirror \( = 4\text{ m} \)
  Distance between boy and his image = distance of boy from the mirror + distance of image from mirror \( = 4 + 4 = 8\text{ m} \).

In simple words: Your image in a flat mirror is always exactly as far behind the glass as you are in front of it. So the gap between you and your reflection is double your distance from the mirror.

📝 Teacher's Note: This property is called "equidistant." The image and object are at the same distance from the mirror plane.

🎯 Exam Tip: Pay attention to whether the question asks for distance from the *mirror* or from the *image*.

Question 35. What is the angle of incidence and reflection for a ray hitting a mirror perpendicularly?
Answer: The angle of incidence = angle of reflection = \( 0^\circ \) because the image is perpendicular to the surface of the plane mirror.
In simple words: If you shine a light straight down onto a mirror, it bounces straight back up. Since it's perfectly aligned with the normal line, the angle is zero.

📝 Teacher's Note: This is the only case where the light ray retraces its own path.

🎯 Exam Tip: "Perpendicular incidence" means the angle of incidence is \( 0^\circ \), not \( 90^\circ \).

Question 36. List two characteristics of an image formed by a plane mirror.
Answer: Two characteristics of image formed by plane mirror are

• Image is erect and virtual
• Image and object are of same size.

In simple words: The reflection is right-side up, it's not a real physical object you can touch, and it's exactly as big as you are.

📝 Teacher's Note: "Virtual" means the light rays don't actually meet at the image; it just *looks* like they are coming from there. "Erect" means it isn't upside down.

🎯 Exam Tip: Use the words "virtual," "erect," and "same size" to earn full marks.

Question 37. Illustrate the lateral inversion of the letter 'P'.
Answer: The reflection takes place at the surface of the plane mirror in accordance with the laws of reflection in which the angle of incidence is equal to angle of reflection. The light coming from the letter P is reflected from the surface of plane mirror. When these reflected rays are produced backwards, they form an inverted virtual image of letter P which is same in size of letter P.
In simple words: When a letter 'P' is placed in front of a mirror, the straight part stays vertical, but the curved bubble flips to the other side.

📝 Teacher's Note: This is the classic example of lateral inversion. Have students try writing their own names so they read correctly in a mirror.

🎯 Exam Tip: In lateral inversion, the image is "perpendicularly flipped" across the mirror line.

Question 38. Show how a beam of light passes through a rectangular opening.
Answer: In simple words: This diagram shows light rays being redirected through a system, changing their path from horizontal to vertical.

📝 Teacher's Note: This could represent a simple light box or the internal workings of an optical instrument like a periscope.

🎯 Exam Tip: Ensure your lines are perfectly straight to represent rectilinear propagation.

Question 39. If a ray of light hits a mirror at a \( 60^\circ \) angle to the normal, find the total angle between the incoming and outgoing rays.
Answer: Since the angle of incidence \( = 60^\circ \). From the diagram, Angle of reflection to the normal = angle of incidence to the normal \( = 60^\circ \). So the angle between incident ray and reflected ray = Angle of incidence + Angle of reflection \( = 60 + 60 = 120^\circ \). 
In simple words: If it hits at \( 60^\circ \), it bounces at \( 60^\circ \). The total turn is both angles added together, which is \( 120^\circ \).

📝 Teacher's Note: This is a simple application of the law of reflection. Make sure students can identify \( i \) and \( r \) on the diagram.

🎯 Exam Tip: The angle between rays is always \( i + r \). Since \( i = r \), it's just \( 2 \times i \).