CBSE Class 7 Science Advanced Physics Light Short Answer Questions Set 01

Short Answer Questions

Teacher's Note:
1. Draw a small table in the exam.
2. Give at least two examples for each.

Question 2. Describe pinhole camera. Write an expression for the magnification of image formed by a pinhole camera.
Answer:
A pinhole camera is a closed box, blackened inside, with a tiny hole in the middle of one face and a screen (paper or glass) on the opposite face.
Light from each point of a bright object travels in a straight line through the hole and falls on the screen. The rays cross at the hole, so a real, inverted image is formed.
Magnification: \( m = \dfrac{\text{size of image}}{\text{size of object}} = \dfrac{h_i}{h_o} = \dfrac{v}{u} \), where v is the distance of the screen from the pinhole and u is the distance of the object from the pinhole.

In simple words: A dark box with a small hole forms an upside-down picture on its back wall.

Teacher's Note:
1. Draw the diagram with u and v marked.
2. The image is real and inverted.

Question 3. (a) What are the characteristics of an image formed by a plane mirror? (b) Write a few uses of plane mirror.
Answer:
(a) 1. The image is of the same size as the object.
2. The image is as far behind the mirror as the object is in front of it.
3. The image is virtual and erect.
4. The image is laterally inverted (left and right are interchanged).
(b) Plane mirrors are used as looking glasses, by opticians to get the proper distance for the eye-testing chart, in shops to make displays look deeper or larger, in solar cookers as reflectors of sunlight, and in kaleidoscopes and periscopes.
In simple words: The image is same size, same distance, upright, virtual and side-reversed.

Teacher's Note:
1. Remember the four characteristics as a list.
2. Give uses from daily life.

Question 4. Describe Newton's disc.
Answer:
Take a circular cardboard disc and divide it into seven equal sectors. Colour them violet, indigo, blue, green, yellow, orange and red. Make a small hole at the centre and fix the disc on a rod.
When the disc turns slowly, the seven colours can be seen separately. When it spins fast, the disc looks white, because the colours blend in our eyes.
This shows that the seven colours together make white light. It was first done by Sir Isaac Newton.

In simple words: A fast-spinning colour disc looks white.

Teacher's Note:
1. Mention that the speed of rotation matters.
2. This is the reverse of dispersion.

Question 5. What are the three rays we consider to draw ray diagrams to show the formation of images by spherical mirrors? How do they propagate after reflection?
Answer:
1. A ray parallel to the principal axis passes through the principal focus after reflection from a concave mirror, and appears to come from the principal focus after reflection from a convex mirror.
2. A ray passing through the principal focus (concave), or directed towards it (convex), becomes parallel to the principal axis after reflection.
3. A ray passing through the centre of curvature (or directed towards it) falls normally on the mirror and retraces its path.

Teacher's Note:
1. Draw all three for both mirrors.
2. Mark C, F and P clearly.

Question 6. Explain how an opaque paper can be made translucent.
Answer:
Rub a little oil (or wax) on the paper. The oil fills the tiny gaps between the fibres, so part of the light now goes through while the rest is scattered. The paper becomes translucent, like oil paper, and objects behind it look blurred.
In simple words: Oiled paper lets some light through.

Teacher's Note:
1. Oil paper is a translucent medium.
2. Dry paper scatters and blocks most of the light.

Question 7. Describe ray of light, beam of light, parallel beam of light, converging beam of light and diverging beam of light.
Answer:
Ray of light: the straight-line path along which light travels, drawn with an arrow.
Beam of light: a group of neighbouring rays.
Parallel beam: rays that keep the same distance from one another (as from a very distant source).
Converging beam: rays that come closer and meet at a point.
Diverging beam: rays that spread out as if coming from a point.

Teacher's Note:
1. Draw all four figures.
2. Converging beams form real images.

Question 8. (a) State laws of reflection. (b) Draw a ray diagram to represent formation of an image of a point source by a plane mirror.
Answer:
(a) 1. The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection (∠i = ∠r).
(b) Two rays OA and OB from the point source O are reflected to A and B and spread out. When extended backwards, the reflected rays meet at O', which is as far behind the mirror as O is in front. O' is the virtual image of O.

In simple words: The reflected rays seem to come from a point behind the mirror, which is the image.

Teacher's Note:
1. Show dotted extensions for the virtual image.
2. OO' is perpendicular to the mirror.

Question 9. Determine the radius of curvature of a convex mirror shown below.

Answer:
The parallel rays diverge after reflection, and their extensions meet at the principal focus F, 10 cm from the pole.
Focal length f = 10 cm.
Radius of curvature R = 2f = 2 × 10 = 20 cm.
In simple words: The radius is twice the distance to the focus.

Teacher's Note:
1. Use R = 2f.
2. The centre of curvature is 20 cm behind the mirror.

Question 10. Which property of light is involved in the formation of shadow? Explain in which case you will not get penumbra at all.
Answer:
Shadows are formed because of the rectilinear propagation of light: light cannot go around an opaque object.
You get no penumbra when the source of light is a point source. Every ray from a single point is blocked or passes, so the shadow is completely dark with a sharp edge. Penumbra needs an extended source whose different points light up the screen partly.
In simple words: Straight-line light makes shadows; a point source gives a shadow with no half-dark edge.

Teacher's Note:
1. Link penumbra with extended sources.
2. A very small source gives a sharp shadow.

Question 11. Why is the light from the Moon called cool light and the light from the Sun called hot light?
Answer:
The Moon does not produce light; it reflects the Sun's light, and the reflected light carries almost no heat to us, so it feels cool.
The Sun is a very hot body that produces light by powerful reactions. Its light comes with a large amount of heat, so it is called hot light (the Sun is a hot source).
In simple words: Moonlight is only reflected sunlight; sunlight is produced by a very hot body.

Teacher's Note:
1. Moon: non-luminous. Sun: luminous hot source.
2. Mention the heat reaching us.

Question 12. During lightning and thundering, why is the lightning seen first and then the thunder heard? Explain.
Answer:
Lightning and thunder are produced at the same moment. Light travels at about 3 × 10⁸ m s⁻¹, whereas sound travels at only about 330 m s⁻¹ in air. So the light reaches us almost at once, and the sound arrives later; we see the flash first and hear the thunder afterwards.
In simple words: Light is far faster than sound.

Teacher's Note:
1. Compare the two speeds.
2. The farther the lightning, the longer the delay.

Teacher's Note:
1. Learn the table.
2. Match the use with the property.

Question 14. What are cold and hot sources of light? Give examples for each.
Answer:
A hot source emits light only when it is very hot, e.g. the Sun, a candle, a filament bulb. A cold source emits light even when it is cool, e.g. a glow-worm.
In simple words: Hot sources glow because of high temperature, cold sources do not need heat.

Teacher's Note:
1. The method of production decides the type.
2. Give two hot and one cold example.

Question 15. Explain how light helps in the growth of plants.
Answer:
Green leaves contain chlorophyll, which absorbs light and converts light energy into chemical energy. In this process, called photosynthesis, carbon dioxide from the air and water combine to form glucose and oxygen is released. The glucose is stored as food. Without light, photosynthesis stops and the plant cannot make its food. Different colours of light affect plant growth differently.
In simple words: Plants use light to make their food.

Teacher's Note:
1. Mention chlorophyll and photosynthesis.
2. Oxygen is released.