CBSE Class 7 Science Advanced Physics Light MCQs Set 03

Answer: (c) BDCEA

Explanation:
1. First mount the mirror on a stand (B).
2. Point it at a distant object (D) and adjust a screen to catch the sharp, inverted, real, diminished image (C).
3. The mirror-to-screen distance is the focal length (E).
4. Finally R = 2f (A). The order is B, D, C, E, A: (c).

Teacher's Note:
1. Set up the apparatus first, take the reading next, calculate last.
2. A distant object sends parallel rays, so the image forms at the focus.

Answer: (d) ACBD

Explanation:
1. Start by noting the glancing angle, 30° (a).
2. Then ∠i = 90° − 30° = 60° (c).
3. The angle of reflection equals the angle of incidence, 60° (b).
4. The angle between the incident and reflected rays is 2 × 60° = 120° (d). The order is a, c, b, d: ACBD.

Teacher's Note:
1. Glancing angle is measured from the mirror surface; the angle of incidence is measured from the normal.
2. The two always add up to 90°.

Answer: (b) R

Explanation:
1. The centre of curvature lies at a distance R from the pole.
2. So the required distance is R, which is also 2f.

Teacher's Note:
1. Pole to focus = f = R/2.
2. Pole to centre of curvature = R = 2f.

Answer: (a) 50°

Explanation:
1. The marked angle ∠d = 80° is the angle of deviation, between the original direction of the ray (dotted line) and the reflected ray.
2. For a plane mirror, ∠i = ∠r and 2∠i + ∠d = 180°.
3. 2∠i = 180° − 80° = 100°, so ∠i = 50°.

Teacher's Note:
1. Angle of deviation d = 180° − 2i.
2. Do not confuse the angle of deviation with the angle of incidence.

Answer: (c) The green-colour light undergoes refraction in prism (DEF) and emerges as a green-colour light.

Explanation:
1. A single colour cannot be split further, so (a) is wrong.
2. White light cannot be made from one colour, so (b) is wrong.
3. The green light only bends (refracts) while passing through the second prism and comes out as green light, so (c) is correct. (d) is wrong because the ray enters the face, it is not reflected back.

Teacher's Note:
1. A pure colour (monochromatic light) is not dispersed again.
2. Seven colours recombine to white only when all of them pass through the second prism.

Answer: (d) Infinite

Explanation:
1. Each mirror makes an image, and each image acts as an object for the other mirror.
2. This repeats endlessly, so the number of images is infinite.

Teacher's Note:
1. For mirrors inclined at θ, n = 360°/θ − 1.
2. Parallel mirrors mean θ = 0°, so n is infinite.

Answer: (d) (A) → b; (B) → c; (C) → a; (D) → e

Explanation:
1. Normal incidence means ∠i = 0°, so ∠r = 0° (A → b).
2. A lunar eclipse forms because light travels in straight lines (B → c).
3. A prism splits white light, which is dispersion (C → a).
4. The moon does not produce its own light, so it is non-luminous (D → e). A burning candle is luminous.
5. Answer (d).

Teacher's Note:
1. A candle makes its own light, so it is luminous.
2. The moon only reflects sunlight.

Answer: (d) All of the above

Explanation:
1. A cement wall allows no light through it: opaque.
2. Distilled water allows light to pass clearly: transparent.
3. Oiled paper lets only part of the light through: translucent.
4. All three are correct: (d).

Teacher's Note:
1. Opaque: no light. Translucent: some light. Transparent: almost all light.
2. Think of wall, oiled paper and clear water.

Answer: (d) Both (b) and (c)

Explanation:
1. The distance from the pole to the principal focus is called the focal length f.
2. For a spherical mirror f = R/2.
3. So both (b) and (c) are correct: (d).

Teacher's Note:
1. f = R/2 for convex and concave mirrors.
2. The radius of curvature is twice the focal length.

Answer: (a) 100°

Explanation:
1. The 40° marked in the figure is between the incident ray and the normal, so ∠i = 40°.
2. ∠r = ∠i = 40°.
3. Angle of deviation d = 180° − 2i = 180° − 80° = 100°.

Teacher's Note:
1. Angles of incidence and reflection are measured from the normal.
2. d = 180° − 2i.

Answer: (a) The size of umbra increases.

Explanation:
1. When the source comes closer to the object, the rays from it spread at larger angles around the object.
2. The shadow (umbra) on the screen becomes larger: (a).

Teacher's Note:
1. A torch brought nearer to your hand makes a bigger shadow on the wall.
2. Point source: only umbra, no penumbra.

Answer: (d) Both (b) and (c)

Explanation:
1. The moon only reflects sunlight, so it is not a source.
2. The sun and a firefly produce their own light and occur in nature.
3. So (d) is correct.

Teacher's Note:
1. Natural sources: sun, stars, fireflies.
2. Artificial sources: candle, bulb.

Answer: (a) The light rays appear to diverge from the principal focus.

Explanation:
1. A convex mirror diverges a parallel beam.
2. The reflected rays seem to come from a point behind the mirror, which is the principal focus: (a).

Teacher's Note:
1. Convex mirror: diverging. Concave mirror: converging.
2. The focus of a convex mirror is virtual.

Answer: (b) BACD

Explanation:
1. Note the distance, 40 cm (B).
2. This distance is the focal length, f = 40 cm (A).
3. Use R = 2f (C).
4. Substitute: R = 2 × 40 = 80 cm (D). Order: B, A, C, D: (b).

Teacher's Note:
1. Distant object: image at the focus.
2. R = 2f.

Answer: (a) BCA

Explanation:
1. First get ∠i = 30° from the data (B).
2. Then the glancing angle of incidence = 90° − 30° = 60° (C).
3. The glancing angle of reflection equals it, 60° (A). Order: B, C, A: (a).

Teacher's Note:
1. Glancing angle = 90° − angle from the normal.
2. Glancing angles of incidence and reflection are equal.

Answer: (b) R/2

Explanation:
1. The principal focus is at f = R/2 from the pole and the centre of curvature is at R.
2. The distance between them is R − R/2 = R/2 (which is also f).

Teacher's Note:
1. Distance F to C = R − f = f.
2. Both F and C lie behind a convex mirror.

Answer: (c) 60°

Explanation:
1. The 60° is marked between the normal and the reflected ray, so ∠r = 60° and ∠i = 60°.
2. Angle of deviation d = 180° − 2i = 180° − 120° = 60°.

Teacher's Note:
1. d = 180° − 2i.
2. Angles are measured from the normal.

Answer: (b) The dispersed light produced by prism (PI) recombines and come out as white light from face (DE) of prism (PII).

Explanation:
1. The second prism is identical but inverted, so it bends the colours back the opposite way.
2. The colours recombine and white light comes out of face DE: (b).
3. Colours already separated cannot be split again, so (a) is wrong.

Teacher's Note:
1. Inverted identical prism: recombination of dispersed light.
2. Newton's experiment showed this.

Answer: (a) 11

Explanation:
1. 360°/θ = 360°/30° = 12, which is an even number.
2. For an even value, the number of images n = 12 − 1 = 11.

Teacher's Note:
1. n = 360°/θ − 1 when 360°/θ is an even number.
2. Check 360°/θ first.

Answer: (c) A → b; B → a; C → d; D → c

Explanation:
1. A solar eclipse is the shadow of the moon falling on the earth (A → b).
2. A glow worm gives light without heat, so it is a cold source (B → a).
3. Magnification = size of the image / size of the object (C → d).
4. A convex mirror gives an erect, virtual and diminished image (D → c).
5. Answer (c).

Teacher's Note:
1. Cold source: gives light without heat.
2. Magnification is image size divided by object size.

Answer: (c) Pure water is a transparent medium.

Explanation:
1. Oil paper is translucent, wood is opaque, and clear glass is transparent, so (a), (b) and (d) are wrong.
2. Pure water is transparent: (c).

Teacher's Note:
1. Opaque: wood, cement. Translucent: oiled paper, frosted glass.
2. Transparent: clear glass, pure water, air.

Answer: (d) Both (a) and (c).

Explanation:
1. The line through the pole and the centre of curvature is itself the principal axis, so (c) is true.
2. Being along the principal axis, it is also parallel to it (a line is parallel to itself in the sense used by the book), so (a) is true and (b) is false.
3. The book's answer is (d).

Teacher's Note:
1. The principal axis passes through the pole and the centre of curvature.
2. This item follows the book's key; (c) alone is the standard statement.

Answer: (c) 60°

Explanation:
1. Treat the surface as a reflecting surface MM¹ (as in the book's solution). The 30° is between the ray and the surface, so ∠i = 90° − 30° = 60°.
2. Angle of deviation d = 180° − 2i = 180° − 120° = 60°.

Teacher's Note:
1. Here the angle is given with the surface, so convert it to the angle with the normal first.
2. d = 2 × glancing angle = 60°.

Answer: (b) The size of umbra decreases.

Explanation:
1. Moving the opaque object away from the point source (towards the screen) makes it receive a narrower cone of light.
2. The umbra on the screen becomes smaller: (b).

Teacher's Note:
1. Object near source: large shadow. Object near screen: small shadow.
2. Same idea as the shadow of your hand on a wall.

Answer: (c) Burning candle

Explanation:
1. The sun, stars and glow worm are natural sources of light.
2. A burning candle is an artificial source, so it is the odd one: (c).

Teacher's Note:
1. Natural versus artificial sources.
2. Candle: man-made.

Answer: (a) Are parallel to the principal axis.

Explanation:
1. A ray directed towards the principal focus of a convex mirror is reflected parallel to the principal axis.
2. So (a).

Teacher's Note:
1. Reverse of a parallel ray appearing to come from F.
2. Ray towards F: reflected parallel. Ray towards C: reflected back along itself.

Answer: (c) Both (A) and (B)

Explanation:
1. Reflection sends light back from a surface, so the direction changes.
2. Refraction bends light when it enters another medium, so the direction changes.
3. Both: (c).

Teacher's Note:
1. Mirror: reflection. Lens or prism: refraction.
2. Light goes straight only in a single uniform medium.

Answer: (a) 12.56 candle power

Explanation:
1. As given in the chapter, 1 lumen = 12.56 candle power.
2. 683 is the number of lumen in one watt, not watt in one lumen, so (b) and (d) are not correct.
3. The answer is (a).

Teacher's Note:
1. 1 watt = 683 lumen.
2. Keep 12.56 and 683 apart.

Answer: (c) Both (a) and (b)

Explanation:
1. The inner surface of a spoon is a concave mirror.
2. A concave mirror gives a magnified virtual image when the object is close to it, and a real image when the object is far away.
3. So both (a) and (b) can happen: (c).

Teacher's Note:
1. Inner side of a spoon: concave. Outer side: convex.
2. Concave mirror images vary with the object position.

Answer: (b) Concave mirror

Explanation:
1. A convex mirror and a plane mirror never form real images.
2. A concave mirror forms a magnified real image when the object is between F and C: (b).

Teacher's Note:
1. Real images need converging rays.
2. Only concave mirrors converge parallel rays.

Answer: (d) ABC

Explanation:
1. Dispersion happens first, at the first surface (A).
2. Then the colours refract again at the second surface (B).
3. Finally the seven-colour spectrum is seen (C). Order: A, B, C: (d).

Teacher's Note:
1. Enter, split, leave, then see the spectrum.
2. Order of the events along the light path.

Answer: (c) P → c; Q → a; R → d; S → b

Explanation:
1. A pin hole camera works because light travels in straight lines (P → c).
2. The umbra is the dark region of a shadow (Q → a).
3. A concave mirror can form a magnified virtual image (R → d).
4. A convex mirror is used as a street light reflector (S → b), as it spreads light over a wide area.
5. Answer (c).

Teacher's Note:
1. Pin hole camera: rectilinear propagation.
2. Convex mirror: wide field of view.

Answer: (b) Infinite

Explanation:
1. Each image acts as an object for the other mirror, and this continues without end.
2. The number of images is infinite.

Teacher's Note:
1. Parallel mirrors: θ = 0°, n = infinite.
2. In practice only a few images can be seen as they get dim.

Answer: (b) 20 cm

Explanation:
1. The radius of curvature of the mirror is R = 40 cm, so f = R/2 = 20 cm.
2. The image of an object at infinity forms at the focus, at 20 cm: (b).

Teacher's Note:
1. Object at infinity: v = f.
2. f = R/2.

Answer: (b) 1 : 1

Explanation:
1. Equal focal lengths: f(convex) = f(concave).
2. R = 2f for both, so R(convex) : R(concave) = 2f : 2f = 1 : 1.

Teacher's Note:
1. R = 2f for any spherical mirror.
2. Same focal length gives same radius.

Answer: (d)

Explanation:
1. (a) Concave mirror: a ray parallel to the axis must go through F, but it is drawn going away from the mirror: wrong.
2. (b) The reflected ray does not pass through F: wrong.
3. (c) Convex mirror: a parallel ray cannot go through F on the other side of the mirror; it must appear to come from F: wrong.
4. (d) Convex mirror: a ray directed towards the centre of curvature C falls normally and is reflected back along its own path: correct.

Teacher's Note:
1. Concave mirror: parallel ray reflects through F.
2. Convex mirror: ray towards C retraces its path; ray towards F reflects parallel.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A convex mirror is used as a rear view mirror, so A is true.
2. It covers a wide field of view and gives a small, erect image, so R is true.
3. These two properties are the reason it is used, so R explains A: (a).

Teacher's Note:
1. A wide view lets the driver see many vehicles behind at once.
2. Diminished image means a large area fits in the mirror.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A rainbow forms when sunlight (white light) falls on raindrops, so A is true.
2. White light has seven colours which are separated by dispersion inside the drop, so R is true.
3. The dispersion is the cause of the rainbow: (a).

Teacher's Note:
1. Rainbow = refraction + dispersion + reflection inside a raindrop.
2. Each raindrop acts like a tiny prism.

Answer: (c) A is true, but R is false.

Explanation:
1. A convex mirror always gives a virtual, erect and diminished image, so A is true.
2. The laws of reflection hold for all mirrors including spherical ones, so R is false.
3. A is true but R is false: (c).

Teacher's Note:
1. Laws of reflection hold at every point of every mirror.
2. Convex mirror: only virtual, erect, diminished images.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Plants make food by photosynthesis in sunlight, so A is true in the book's sense.
2. Light energy is needed, so R is true.
3. Because light is available only in the day, R explains A: (a).

Teacher's Note:
1. Light is changed into chemical energy with the help of chlorophyll.
2. Natural photosynthesis needs sunlight.