CBSE Class 10 Physics Light Reflection and Refraction MCQs Set 13

Answer: (c) deviate towards the normal

Explanation:
1. Air is optically rarer than water.
2. A ray entering a denser medium obliquely slows down and bends towards the normal.

Teacher's Note:
1. Rarer to denser: bends towards the normal.
2. Denser to rarer: bends away from the normal.

Answer: (b) Optically denser

Explanation:
1. A medium with a larger refractive index is called optically denser.
2. A medium with a smaller refractive index is optically rarer.

Teacher's Note:
1. Optical density is different from mass density.
2. Compare refractive indices, not masses.

Answer: (a) A

Explanation:
1. Speed of light in a medium v = c/n.
2. Speed is greatest where n is least, i.e. medium A (n = 1.33).

Teacher's Note:
1. Larger refractive index means slower light.
2. Compare the numbers and pick the smallest n.

Answer: (c) 1.32

Explanation:
1. n = (speed of light in air)/(speed of light in water).
2. n = (3 × 10⁸)/(2.26 × 10⁸) = 1.327 ≈ 1.32.

Teacher's Note:
1. Keep air (or vacuum) speed in the numerator.
2. The refractive index has no unit.

Answer: (d) 2

Explanation:
1. n = c/v = (3 × 10⁸)/(1.5 × 10⁸) = 2.

Teacher's Note:
1. The ratio of the speeds gives n directly.
2. The units cancel, so n has no unit.

Answer: (b) 1.21

Explanation:
1. n_{glass/alcohol} = n_glass/n_alcohol = 1.65/1.36 = 1.213.
2. So the answer is 1.21.

Teacher's Note:
1. Divide the index of the second medium (glass) by that of the first (alcohol).
2. Do not subtract the indices.

Answer: (b) 1.80

Explanation:
1. n_{diamond/water} = n_diamond/n_water = 2.4/1.33 = 1.80.

Teacher's Note:
1. Relative index = (index of the second medium)/(index of the first medium).
2. Here light goes from water to diamond.

Answer: (b) 2/2.5

Explanation:
1. n₁₂ = (speed of light in medium 2)/(speed of light in medium 1).
2. n₁₂ = (2 × 10⁸)/(2.5 × 10⁸) = 2/2.5.

Teacher's Note:
1. Refractive index of 1 with respect to 2 is v₂/v₁.
2. The medium written second goes in the numerator for speeds.

Answer: (a) 0°

Explanation:
1. At normal incidence i = 0°, the ray goes straight and r = 0°.
2. For any other angle, sin i/sin r = n ≠ 1 so r differs from i.

Teacher's Note:
1. Normal incidence means no bending.
2. Do not confuse the angle with the surface and the angle with the normal.

Answer: (a) Difference in density

Explanation:
1. Light travels at different speeds in media of different optical density.
2. Glass is optically denser than air, so light is slower in it.

Teacher's Note:
1. The cause of refraction is the change in the speed of light.
2. Optical density, not wind or brightness, decides the speed.

Answer: (b) A-B

Explanation:
1. Bending is least where the ratio of the two refractive indices is closest to 1.
2. air-A: 1.50/1.0 = 1.50; A-B: 1.33/1.50 = 0.89 (ratio 1.13); B-C: 2.42/1.33 = 1.82; C-air: 1/2.42 (ratio 2.42).
3. The smallest change is for A-B.

Teacher's Note:
1. Compare the ratios of refractive indices, not the media names.
2. Bigger difference in n means bigger bending.

Answer: (c) The speed of light in air > the speed of light in water > the speed of light in glass.

Explanation:
1. Speed v = c/n, so a smaller refractive index means a higher speed.
2. n(air) = 1.0003 < n(water) = 1.333 < n(glass) = 1.52.
3. Speeds: air > water > glass.

Teacher's Note:
1. Order the refractive indices first, then reverse the order for speeds.
2. Light is slowest in the optically densest medium.

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

Explanation:
1. Frequency stays constant when light changes medium, and v = fλ.
2. In a denser medium v decreases, so λ = v/f also decreases.
3. So both velocity and wavelength decrease.

Teacher's Note:
1. Frequency is fixed by the source.
2. Speed and wavelength change together.

Answer: (b) 4000 Å

Explanation:
1. Wavelength in vacuum λ = c/f = (3 × 10⁸)/(5 × 10¹⁴) = 6 × 10⁻⁷ m = 6000 Å.
2. In the medium λ′ = λ/n = 6000/1.5 = 4000 Å.

Teacher's Note:
1. Frequency does not change when light enters a new medium.
2. Wavelength becomes λ/n.

Answer: (a) Wavelength and velocity

Explanation:
1. Frequency is fixed by the source and does not change.
2. Speed changes with the medium, and since v = fλ, the wavelength changes too.

Teacher's Note:
1. Only wavelength and speed change.
2. Frequency of light always remains the same.

Answer: (c) III

Explanation:
1. In a rectangular slab ∠e = ∠i, so statement I is wrong.
2. Also ∠e = ∠i > ∠r, so II is wrong.
3. The emergent ray is parallel to the incident ray (III is correct) but not to the refracted ray (IV is wrong).

Teacher's Note:
1. The slab shifts the ray sideways without changing its direction.
2. The refracted ray inside the slab has a different direction.

Answer: (d) gets deviated and bends towards the thicker part (base) of the prism.

Explanation:
1. The two faces of a prism are inclined, so the ray bends at both faces in the same sense.
2. The emergent ray is deviated towards the base (thicker part) of the prism.

Teacher's Note:
1. In a prism the emergent ray is not parallel to the incident ray.
2. It always bends towards the base.

Answer: (c) 2 cm

Explanation:
1. Apparent depth = real depth/refractive index.
2. Apparent depth = 3 cm/1.5 = 2 cm below the upper surface.

Teacher's Note:
1. A slab makes things look raised by t(1 − 1/n) = 1 cm here.
2. Apparent depth = real depth ÷ n.

Answer: (a) 5.32 metres

Explanation:
1. Apparent depth = real depth/n, so real depth = apparent depth × n.
2. Real depth = 4 × 1.33 = 5.32 m.

Teacher's Note:
1. Real depth is greater than apparent depth in water.
2. Multiply by n, do not add.

Answer: (a) ‘l’ keeps on increasing with increase in angle of incidence

Explanation:
1. The perpendicular distance l between the incident and emergent rays is the lateral displacement.
2. Lateral displacement = t sin(i − r)/cos r, which increases as the angle of incidence increases.
3. So l keeps increasing with ∠i.

Teacher's Note:
1. Lateral shift depends on the thickness of the slab and the angle of incidence.
2. It is zero for normal incidence.

Answer: (b) Black absorbs all the colours

Explanation:
1. Red cloth can reflect red light, but under green light (which it absorbs) it looks black.
2. Black cloth absorbs all colours, so it has no light to reflect.
3. Hence it never looks red.

Teacher's Note:
1. The colour of an object is the colour it reflects.
2. Black absorbs all colours; white reflects all.

Answer: (b) 1.55

Explanation:
1. The glass piece is invisible when light does not bend or reflect at its surface.
2. That happens when its refractive index equals that of the liquid, i.e. 1.55.

Teacher's Note:
1. Equal refractive indices mean no refraction at the boundary.
2. This is why a glass rod disappears in a liquid of the same refractive index.

Answer: (b) 1.67 × 10⁸ ms⁻¹

Explanation:
1. v = c/n = (3 × 10⁸)/1.8 = 1.67 × 10⁸ ms⁻¹.
2. Check with crown glass: 3 × 10⁸/1.5 = 2 × 10⁸ ms⁻¹, as given.

Teacher's Note:
1. Higher n means lower speed: 1.8 > 1.5, so the speed must be less than 2 × 10⁸.
2. Use v = c/n.

Answer: (a) 1.5 × 10⁻⁵ s

Explanation:
1. Speed in glass v = c/n = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s.
2. Time t = distance/speed = 3000/(2 × 10⁸) = 1.5 × 10⁻⁵ s.

Teacher's Note:
1. Convert 3 km to 3000 m before dividing.
2. Use the speed in glass, not in vacuum.

Answer: (b) below the image of fish

Explanation:
1. The fish appears raised, so its image is above the real fish.
2. The real fish is below the image, so the hunter must aim below the image.

Teacher's Note:
1. Objects in water look closer to the surface than they are.
2. Aim below what you see.

Answer: (d) 5/3

Explanation:
1. The dot appears 2.0 cm above the bottom, so apparent depth = 5.0 − 2.0 = 3.0 cm.
2. n = real depth/apparent depth = 5.0/3.0 = 5/3.

Teacher's Note:
1. Apparent depth is measured from the top surface.
2. n = real depth ÷ apparent depth is always greater than 1.

Answer: (a) increases

Explanation:
1. The refractive index of glass (about 1.5) is greater than that of water (1.33).
2. Glass is optically denser, so light moves from a denser medium to a rarer one and its speed increases.

Teacher's Note:
1. Compare the refractive indices to decide the change of speed.
2. Speed rises when going to a lower n.

Answer: (a) shallower

Explanation:
1. Light rays from the bottom bend away from the normal as they leave the water.
2. The bottom therefore appears raised, i.e. the pool appears shallower.

Teacher's Note:
1. Apparent depth = real depth ÷ 1.33.
2. A swimming pool is deeper than it looks.

Answer: (a) Kerosene

Explanation:
1. A medium with a refractive index greater than that of water (1.33) is optically denser.
2. Kerosene (1.44) is denser; air (1.0003) and ice (1.31) are rarer.

Teacher's Note:
1. Compare the refractive indices from the table.
2. Higher n means optically denser.

Answer: (b) 1.97 × 10⁸ ms⁻¹

Explanation:
1. v = c/n = (3 × 10⁸)/1.52 = 1.97 × 10⁸ ms⁻¹.

Teacher's Note:
1. Speed in glass is always less than in air.
2. Divide c by n.

Answer: (d) 1.501

Explanation:
1. An optically denser medium has a higher refractive index.
2. The refractive index of benzene must therefore be greater than 1.361, and only 1.501 satisfies this.

Teacher's Note:
1. Compare the options with 1.361.
2. Benzene's actual refractive index is about 1.50.

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

Explanation:
1. The refractive index of glass is smaller for red light than for green light, so Reason is true.
2. Speed in glass v = c/n, and a smaller n means a higher speed.
3. So red light travels faster, and R explains A.

Teacher's Note:
1. Different colours have different speeds in glass; this is the reason for dispersion.
2. Red light bends least and travels fastest.

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

Explanation:
1. The frequency of light is decided by its source and stays the same in all media.
2. So the assertion is true and the reason explains it.

Teacher's Note:
1. Speed and wavelength change when light changes medium, but frequency does not.
2. v = fλ with f constant.

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

Explanation:
1. Glycerine has the highest refractive index among these (about 1.47), so it bends light the most: A is true.
2. R is false, because glycerine does not have the lowest refractive index.

Teacher's Note:
1. More refraction means a higher refractive index.
2. Compare the values: 1.33, 1.44, 1.46, 1.47.

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

Explanation:
1. Light slows down on entering a denser medium, so A is false.
2. Light travels faster in a rarer medium, so R is true.

Teacher's Note:
1. Denser medium means lower speed.
2. Rarer medium means higher speed.

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

Explanation:
1. Light travels slower in glass than in air, so A is false.
2. Glass is optically denser than air, so R is true.

Teacher's Note:
1. Denser medium means slower light.
2. The reason does not support the assertion; it contradicts it.

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

Explanation:
1. n = (speed of light in vacuum)/(speed in medium), a ratio of two speeds.
2. The units cancel, so n has no unit.
3. Both are true and R explains A.

Teacher's Note:
1. A ratio of like quantities is a pure number.
2. Other examples: magnification and relative density.

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

Explanation:
1. n = c/v, so n is inversely proportional to v.
2. A higher n therefore means a lower speed.
3. Both are true and R explains A.

Teacher's Note:
1. Memorise v = c/n.
2. Greater n, smaller v.

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

Explanation:
1. Water is denser than air, so the speed of light decreases when it passes from air to water: A is false.
2. R is true.

Teacher's Note:
1. Speed decreases when entering a denser medium.
2. Do not reverse the direction.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Kerosene (1.44) has a higher refractive index than water (1.33), so it is optically denser: A is true.
2. Kerosene is lighter than water, so R is true.
3. Optical density is different from mass density, so R does not explain A.

Teacher's Note:
1. Optical density depends on the refractive index, not on mass density.
2. Kerosene floats on water yet is optically denser.