The Human Eye and the Colourful World: Class 10 Important Questions with Answers

Case-based questions (4 marks each)

(i) Name Rohan's defect and its two causes. (1) (ii) Find the power of the lens Rohan needs. (1) (iii) Name the grandfather's defect, its cause and the lens; what if he also had Rohan's defect? (2)

(i) Name the band and the phenomenon. (1) (ii) Which colour bends most and which least? (1) (iii) What does the second prism show? Explain. (2)

(i) Name the phenomenon behind both observations. (1) (ii) Why does Jupiter not twinkle? (1) (iii) Explain why stars twinkle and why the Sun is seen before actual sunrise. (2)

(i) Name the phenomenon. (1) (ii) Why are the red lights seen better in fog? (1) (iii) Why does the Sun look red at sunset but white at noon? (2)

20 MCQs (1 mark each)

1. Most refraction in the eye occurs at the
   (a) lens (b) cornea (c) retina (d) pupil
2. The size of the pupil is controlled by the
   (a) cornea (b) iris (c) retina (d) optic nerve
3. The image on the retina is
   (a) virtual, erect (b) real, inverted (c) real, erect (d) virtual, inverted
4. For distant objects the ciliary muscles are
   (a) contracted (b) relaxed (c) torn (d) absent
5. Far point of a normal eye:
   (a) 25 cm (b) 1 m (c) 100 m (d) infinity
6. Milky, cloudy eye lens in old age:
   (a) myopia (b) cataract (c) presbyopia (d) hypermetropia
7. In myopia the image forms
   (a) on the retina (b) behind it (c) in front of it (d) at the lens
8. Hypermetropia is corrected by a
   (a) concave lens (b) convex lens (c) plane glass (d) cylindrical lens
9. An elongated eyeball causes
   (a) myopia (b) hypermetropia (c) cataract (d) presbyopia
10. In bifocal lenses the upper part is
    (a) convex (b) concave (c) plane (d) prism
11. Power needed for far point 50 cm (myopia):
    (a) +2 D (b) −2 D (c) −0.5 D (d) +0.5 D
12. The colour that deviates most in a prism:
    (a) red (b) yellow (c) green (d) violet
13. VIBGYOR starts from the colour that bends
    (a) least (b) most (c) not at all (d) randomly
14. A rainbow is seen
    (a) towards the Sun (b) opposite to the Sun (c) only at noon (d) at night
15. Stars twinkle due to
    (a) dispersion (b) atmospheric refraction (c) scattering (d) reflection
16. Advance sunrise is about
    (a) 2 s (b) 2 min (c) 20 min (d) 2 h
17. The blue colour of the sky is due to
    (a) reflection (b) dispersion (c) scattering (d) refraction
18. Danger signals are red because red is
    (a) most scattered (b) least scattered (c) brightest (d) cheapest
19. Tyndall effect is shown by
    (a) salt solution (b) sugar solution (c) milk diluted / smoke (d) pure water
20. The sky looks dark to an astronaut because
    (a) no Sun (b) no scattering (c) eyes adjust (d) the Moon blocks light

Answer key

Why: 11 → f = −0.5 m, P = −2 D · 13 → V (violet) bends the most and is at the bottom.

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: A normal eye can see objects from 25 cm to infinity. R: The eye can change the focal length of its lens. → (a)
2. A: Myopia is corrected by a convex lens. R: In myopia the image forms in front of the retina. → (d)
3. A: Presbyopia occurs in old age. R: Ciliary muscles weaken and the lens loses flexibility. → (a)
4. A: White light splits into colours in a prism. R: Different colours have different speeds in glass and bend differently. → (a)
5. A: The rainbow forms opposite to the Sun. R: Water droplets act like tiny prisms. → (b)
6. A: Planets do not twinkle. R: Planets are point-sized sources. → (c)
7. A: The sky appears blue. R: Blue light is scattered least. → (c)
8. A: Danger signals are red. R: Red light has the longest wavelength and is least scattered. → (a)
9. A: The Sun appears flattened at sunset. R: Atmospheric refraction near the horizon. → (a)
10. A: A ray of light through a glass slab emerges parallel. R: A glass slab disperses white light into a spectrum. → (c)

Give reasons (2 marks each)

1. We cannot see clearly an object held very close to the eye.

The eye lens cannot become thick enough to reduce its focal length below a limit; objects closer than 25 cm form a blurred image. 2

2. The pupil becomes small in bright light.

The iris contracts the pupil to let in less light and protect the retina. 2

3. A beam of light is visible in a smoky room.

Smoke particles scatter the light into our eyes — Tyndall effect. 2

4. The Sun appears reddish at sunrise.

Near the horizon sunlight travels a longer path in air; blue is scattered away and mostly red reaches us. 2

5. Hot air above a fire makes objects behind it waver.

Hot air has a lower refractive index and keeps moving, so refraction through it keeps changing the apparent position — atmospheric refraction. 2