CBSE Class 10 Physics Human Eye and Colourful World MCQs Set 09

Answer: (b) cornea

Explanation:
1. Light entering the eye from air is refracted mostly at the outer surface of the cornea; the lens only fine-tunes the focus.

Teacher's Note:
1. The retina is the screen.

Answer: (a) iris

Explanation:
1. The iris is a muscular diaphragm; it narrows the pupil in bright light and widens it in dim light.

Teacher's Note:
1. The colour of the eye is the colour of the iris.

Answer: (d) real and inverted

Explanation:
1. The eye lens is a convex lens; the brain interprets the inverted image as upright.

Teacher's Note:
1. Retina contains rods and cones.

Answer: (c) relaxed; thin

Explanation:
1. A thin lens has a larger focal length, which is what distant objects need to focus on the retina.

Teacher's Note:
1. For near objects the lens becomes thicker.

Answer: (b) accommodation

Explanation:
1. Ciliary muscles change the curvature of the lens to focus on the retina.

Teacher's Note:
1. It decreases with age.

Answer: (a) 25 cm

Explanation:
1. This is the near point of a normal eye, at which an object can be seen clearly without strain.

Teacher's Note:
1. The far point of a normal eye is at infinity.

Answer: (d) infinity

Explanation:
1. A normal eye can see very distant objects clearly.

Teacher's Note:
1. The near point is about 25 cm.

Answer: (c) blind spot

Explanation:
1. There are no light-sensitive cells at this spot, so an image formed there is not seen.

Teacher's Note:
1. The macula lutea is the region of sharpest vision.

Answer: (b) respond to colours and work in bright light

Explanation:
1. Cones are sensitive to red, green and blue; rods work in dim light and cannot distinguish colours.

Teacher's Note:
1. Colour blindness is due to defective cones.

Answer: (a) myopia

Explanation:
1. In myopia the image of a distant object is formed in front of the retina because the eye lens is too powerful or the eyeball is too long.

Teacher's Note:
1. It is also called near-sightedness.

Answer: (d) concave lens

Explanation:
1. A diverging lens makes the parallel rays from distant objects diverge so that the image shifts back onto the retina.

Teacher's Note:
1. The lens power is negative.

Answer: (c) hypermetropia

Explanation:
1. The image of a near object forms behind the retina because the focal length of the eye lens is too long or the eyeball is too short.

Teacher's Note:
1. It is corrected by a convex lens.

Answer: (b) convex lens

Explanation:
1. A converging lens reduces the divergence so that the image falls on the retina.

Teacher's Note:
1. The power of the corrective lens is positive.

Answer: (a) weakening of the ciliary muscles and loss of flexibility of the eye lens

Explanation:
1. The near point recedes with age, so reading becomes difficult.

Teacher's Note:
1. It is corrected with convex (or bifocal) lenses.

Answer: (d) -1.25 D

Explanation:
1. The lens must form a virtual image of a distant object at the far point: f = -80 cm = -0.8 m, so P = 1/f = -1.25 D.

Teacher's Note:
1. A concave lens is used.

Answer: (c) +37.5 cm and +2.67 D

Explanation:
1. With u = -0.25 m and v = -0.75 m, 1/f = 1/v - 1/u = -1.333 + 4 = 2.667 per metre. So f = 0.375 m and P = +2.67 D.

Teacher's Note:
1. The lens brings the book's image to the near point.

Answer: (b) colour blindness

Explanation:
1. It results from missing or defective cone cells for particular colours and is a genetic disorder.

Teacher's Note:
1. Red-green colour blindness is common.

Answer: (a) cataract

Explanation:
1. It can be cured by surgery.

Teacher's Note:
1. The lens develops a hazy membrane.

Answer: (d) Red

Explanation:
1. Red has the longest wavelength and the smallest refractive index in glass, so it bends the least.

Teacher's Note:
1. Violet bends the most.

Answer: (c) dispersion

Explanation:
1. The refractive index of glass differs for different wavelengths, so colours bend by different amounts.

Teacher's Note:
1. Newton showed this using a second inverted prism.

Answer: (b) the emergent rays of all colours are parallel and recombine to give white light

Explanation:
1. Refraction at the second parallel face reverses the first: the colours emerge parallel and overlap, so only slight coloured edges appear.

Teacher's Note:
1. A prism's faces are inclined, so colours separate.

Answer: (a) Red

Explanation:
1. Red has the lowest refractive index in glass, so its speed is the highest.

Teacher's Note:
1. Violet travels slowest.

Answer: (d) dispersion, internal reflection and refraction of sunlight by water droplets

Explanation:
1. Sunlight is refracted and dispersed on entering a drop, reflected internally and refracted again on leaving.

Teacher's Note:
1. It forms when the Sun is behind the observer.

Answer: (c) atmospheric refraction of starlight

Explanation:
1. As starlight passes through layers of the atmosphere of varying refractive index, its apparent position fluctuates.

Teacher's Note:
1. Planets do not twinkle as they are extended sources.

Answer: (b) shorter wavelengths of sunlight are scattered more by air molecules

Explanation:
1. Scattering is stronger for shorter wavelengths, so blue light reaches our eyes from all directions.

Teacher's Note:
1. Rayleigh scattering varies inversely as the fourth power of wavelength.