CBSE Class 10 Physics Human Eye and the Colourful World MCQs Set 10

Answer: (b) atmospheric refraction

Explanation:
1. Atmospheric refraction makes the Sun visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.
2. Together these give about 4 minutes.

Teacher's Note:
1. The Sun's disc also looks flattened at sunrise and sunset.

Answer: (a) real and inverted

Explanation:
1. The eye lens forms a real, inverted image on the retina.
2. The brain interprets it as erect.

Teacher's Note:
1. Both real and inverted. Not the same as the image seen in a plane mirror.

Answer: (b) decreases

Explanation:
1. Air is denser at lower altitudes and becomes rarer higher up, so its refractive index decreases with height.

Teacher's Note:
1. This variation causes atmospheric refraction effects.

Answer: (c) cylindrical

Explanation:
1. Astigmatism arises because the cornea is not equally curved in all directions; a cylindrical lens compensates it.

Teacher's Note:
1. Astigmatism can occur with myopia or hypermetropia.

Answer: (b) B and D

Explanation:
1. In a denser medium light travels slower and the refractive index is higher, so B is true and A is false.
2. A ray entering a denser medium bends towards the normal: going from 2 (rarer) to 1 (denser), D is true, and C is false.

Teacher's Note:
1. Rarer to denser: towards the normal.

Answer: (c) True

Explanation:
1. Very fine particles scatter mostly blue light; larger particles scatter longer wavelengths and the scattered light may appear whitish.

Teacher's Note:
1. The Tyndall effect makes a beam visible in a dusty room.

Answer: (a) Their long wavelength is scattered least by fog

Explanation:
1. Red and orange have long wavelengths, so they are scattered much less by small particles and travel farther through fog or smoke.

Teacher's Note:
1. Blue light would be scattered strongly and fade quickly.

Answer: (c) presbyopia

Explanation:
1. Presbyopia is the loss of the power of accommodation with age.

Teacher's Note:
1. A bifocal lens can correct it together with myopia.

Answer: (c) 1.41

Explanation:
1. The ray meets the hypotenuse at an angle of incidence of 45 degrees.
2. Total internal reflection needs 45 degrees to be at least the critical angle, so \( \mu \ge 1/\sin 45^\circ = \sqrt{2} = 1.41 \).

Teacher's Note:
1. Ordinary glass (about 1.5) satisfies this.

Answer: (b) a concave lens of focal length 1.2 m

Explanation:
1. The lens must form images of distant objects at the person's far point, 1.2 m, so it is a diverging lens.
2. \( 1/f = 1/v - 1/u = -1/1.2 \), so f = -1.2 m, about -0.83 D.

Teacher's Note:
1. Myopia is corrected with a concave lens.

Answer: (b) 0.12/3 rad

Explanation:
1. \( \theta = 0.12/3 = 0.04 \) rad.

Teacher's Note:
1. Use the on-screen height and the viewer's distance.

Answer: (c) The cornea and aqueous humour

Explanation:
1. The outer surface of the cornea, together with the aqueous humour behind it, bends light most because of the large change in refractive index at the air-cornea surface.

Teacher's Note:
1. The lens only does the fine focusing.

Answer: (c) It remains the same

Explanation:
1. The distance between the eye lens and the retina is fixed.
2. The lens changes its focal length so that the image is always on the retina.

Teacher's Note:
1. This adjustment is called accommodation.

Answer: (a) myopia, -2 D

Explanation:
1. A concave (negative) lens corrects myopia.
2. \( P = 1/f = 1/(-0.5) = -2 \) D.

Teacher's Note:
1. Near-sightedness: distant objects look blurred.

Answer: (b) The TV viewer

Explanation:
1. TV viewer: 0.04 rad; spectator: 0.025 rad.
2. The larger angle gives a larger apparent size, so the TV viewer sees the bigger player.

Teacher's Note:
1. Apparent size depends on the angle subtended.

Answer: (b) It emerges as white light

Explanation:
1. The second prism recombines the colours separated by the first, so white light emerges.

Teacher's Note:
1. Newton used this experiment to show that a prism only separates colours.

Answer: (a) A-R, B-P, C-S, D-Q

Explanation:
1. Scattering intensity goes as 1/lambda^4 (A-R).
2. The prism equation is i + e = A + delta (B-P).
3. Magnifying power of a simple microscope is 1 + D/f (C-S).
4. Critical angle gives mu = 1/sin C for total internal reflection (D-Q).

Teacher's Note:
1. Remember each formula with its phenomenon.

Answer: (a) 1.9/75 rad

Explanation:
1. \( \theta = 1.9/75 \approx 0.025 \) rad.

Teacher's Note:
1. Angular size = height divided by distance.

Answer: (b) a nearby object

Explanation:
1. For a near object the ciliary muscles contract, the lens becomes thicker and its focal length decreases.
2. For a distant object the lens is thin and the focal length is largest.

Teacher's Note:
1. The distance between the lens and the retina stays fixed.

Answer: (b) large, very large

Explanation:
1. A telescope is used to see large objects (the Moon, stars, buildings) at very large distances.

Teacher's Note:
1. A microscope is for tiny near objects.

Answer: (c) Blue

Explanation:
1. Scattering by very small particles follows \( I \propto 1/\lambda^4 \).
2. Blue light, with the shorter wavelength, is scattered the most.

Teacher's Note:
1. That is why the sky appears blue.

Answer: (b) The ciliary muscles weaken and the lens loses flexibility, so the near point recedes

Explanation:
1. With age the ciliary muscles become weak and the eye lens becomes less flexible.
2. The eye cannot reduce its focal length enough, causing presbyopia.

Teacher's Note:
1. Presbyopia is corrected with bifocal or convex lenses for reading.

Answer: (d) both reducible and increasable

Explanation:
1. The ciliary muscles can make the lens thinner (longer f) or thicker (shorter f), so the focal length can both increase and decrease.

Teacher's Note:
1. This adjustment is accommodation.