CBSE Class 10 Physics Human Eye and the Colourful World MCQs Set 32

Answer: (a) 2.5 cm

Explanation:
1. The eye lens forms the image on the retina about 2.5 cm behind it, so its focal length is about 2.5 cm for distant objects.

Teacher's Note:
1. Distance from lens to retina is about 2.5 cm.

Answer: (b) total internal reflection, refraction and dispersion of light through the water droplets

Explanation:
1. Sunlight is refracted and dispersed on entering a raindrop, totally internally reflected inside it, and refracted again on leaving.
2. So all three processes are involved.

Teacher's Note:
1. Rainbow needs sunshine behind you and rain in front.

Answer: (c) violet

Explanation:
1. In the visible spectrum, violet has the shortest wavelength (about 400 nm) and red the longest.

Teacher's Note:
1. VIBGYOR: wavelength increases from violet to red.

Answer: (b) speed of light

Explanation:
1. Different colours travel with different speeds in glass, so their refractive indices differ and they bend by different amounts.
2. Frequency does not change on entering a medium.

Teacher's Note:
1. Frequency depends only on the source.

Answer: (c) behind the retina

Explanation:
1. A long-sighted eye is too short or its lens too weak, so the image of a nearby object falls behind the retina.

Teacher's Note:
1. Myopia forms the image in front of the retina.

Answer: (d) astigmatism

Explanation:
1. An uneven curvature of the cornea gives different focal lengths in different planes, which is astigmatism (corrected by a cylindrical lens).

Teacher's Note:
1. Astigmatism is corrected by cylindrical lenses.

Answer: (b) scattering of light

Explanation:
1. Air molecules scatter short wavelengths (blue) much more strongly than long ones, so the scattered blue light reaches our eyes from all directions.

Teacher's Note:
1. Scattering depends on wavelength: blue scatters most.
2. Red is scattered least.

Answer: (c) +60 cm

Explanation:
1. The lens must form a virtual image of the book at 120 cm: u = –40 cm and v = –120 cm.
2. 1/f = 1/v – 1/u = –1/120 + 1/40 = 2/120, so f = +60 cm (convex).

Teacher's Note:
1. Far-sighted needs a convex lens.

Answer: (c) ciliary muscles

Explanation:
1. The ciliary muscles change the curvature, and so the focal length, of the eye lens.

Teacher's Note:
1. Iris controls the pupil size.
2. Ciliary muscles control the lens.

Answer: (c) 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, red the least.

Answer: (d) retina

Explanation:
1. The eye lens focuses the image on the retina, where light-sensitive cells send signals to the brain.

Teacher's Note:
1. Retina is the screen of the eye.

Answer: (c) a bifocal lens

Explanation:
1. Presbyopia (loss of accommodation with age) often needs both distance and near correction, which a bifocal lens provides.

Teacher's Note:
1. Bifocal lens: concave on top, convex below.

Answer: (b) less than P

Explanation:
1. The relative refractive index of glass with respect to water is smaller than that with respect to air.
2. So the lens bends light less, and its power decreases.

Teacher's Note:
1. Power depends on the surrounding medium.

Answer: (c) contract, decrease

Explanation:
1. For nearby objects the muscles contract, the lens becomes thicker (more curved), and its focal length decreases.

Teacher's Note:
1. Thicker lens means shorter focal length.
2. This is called accommodation.

Answer: (a) black

Explanation:
1. Without an atmosphere there is nothing to scatter sunlight towards us, so the sky would look black.

Teacher's Note:
1. The sky looks black from the moon.

Answer: (c) change in the convexity of the lens

Explanation:
1. The ciliary muscles change the curvature (convexity) of the lens, which alters its focal length.

Teacher's Note:
1. Camera moves the lens, eye changes its shape.

Answer: (c) 25 cm

Explanation:
1. The near point of a normal eye is about 25 cm.

Teacher's Note:
1. Far point is infinity.

Answer: (c) a concave lens as the upper part and a convex lens as the lower part

Explanation:
1. For distant vision (myopia) the upper part is concave, and for reading (hypermetropia) the lower part is convex.
2. This is a bifocal lens.

Teacher's Note:
1. Upper part: distance vision. Lower part: near vision.

Answer: (b) (i)-B, (ii)-C, (iii)-A, (iv)-D

Explanation:
1. Rainbow is due to dispersion (B).
2. Twinkling is due to fluctuation of refractive index in atmospheric layers (C).
3. Blue sky is due to scattering (A).
4. Early sunrise and late sunset are due to atmospheric refraction (D).

Teacher's Note:
1. Atmospheric refraction raises the apparent position of the sun.

Answer: (b) long sightedness

Explanation:
1. A long-sighted eye focuses the image behind the retina, and a convex lens converges the light so that the image comes forward onto the retina.

Teacher's Note:
1. Myopia: concave lens. Hypermetropia: convex lens.