Class 10 Science Chapter 10, The Human Eye and the Colourful World, has 17 end-of-chapter exercise questions in the current 2026-27 rationalised session (this chapter shifted down from the older Chapter 11 numbering in pre-rationalisation editions — confirmed via multiple independent sources before writing). Below are original, concept-based solutions to all 17.
Why This Chapter Matters for Boards
This chapter blends the Human Eye (a high-scoring biology-meets-physics topic covering myopia, hypermetropia and lens power numericals) with Atmospheric Refraction and Scattering of Light (twinkling of stars, reddish sunrise/sunset, blue sky). Both numerical and reasoning-based questions are common in the CBSE board exam.
Exercise Solutions
- Q1. What is meant by power of accommodation of the eye?
The ability of the eye lens to adjust its focal length, by the action of the ciliary muscles, so that objects at different distances can be focused clearly on the retina. This is called the power of accommodation. - Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What is the type of corrective lens used to restore proper vision?
This is myopia (near-sightedness); the far point is 1.2 m instead of infinity. A concave (diverging) lens of focal length f = −1.2 m is required. Power = 1/f = 1/(−1.2) = −0.83 D (approx.) - Q3. What is the far point and near point of the human eye with normal vision?
Far point: infinity. Near point: about 25 cm from the eye. - Q4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
The child likely has myopia (cannot see distant objects clearly). It can be corrected using spectacles with a suitable concave lens, or by consulting an eye specialist for the exact power required; sitting closer temporarily helps but is not a correction. - Q5. (MCQ) The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to:
(a) presbyopia (b) accommodation (c) near-sightedness (d) far-sightedness
Answer: (b) accommodation — achieved by the ciliary muscles changing the curvature of the eye lens. - Q6. (MCQ) The human eye forms the image of an object at its:
(a) cornea (b) iris (c) pupil (d) retina
Answer: (d) retina - Q7. (MCQ) The least distance of distinct vision for a young adult with normal vision is about:
(a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m
Answer: (c) 25 cm - Q8. (MCQ) The change in focal length of an eye lens is caused by the action of the:
(a) pupil (b) retina (c) ciliary muscles (d) iris
Answer: (c) ciliary muscles — they contract or relax to change the curvature (and hence focal length) of the eye lens. - Q9. A person needs a lens of power +1.5 D for correcting distant vision… wait – correction for near vision, and −5.5 D for correcting distant vision. Find the focal length of each lens.
Near vision lens: f = 1/P = 1/1.5 = +0.67 m (converging — hypermetropia correction)
Distant vision lens: f = 1/(−5.5) = −0.18 m (diverging — myopia correction). This person needs bifocal lenses. - Q10. The far point of a myopic person is 80 cm in front of the eye. What is the power of the lens required to enable him to see distant objects clearly?
f = −0.8 m. P = 1/f = 1/(−0.8) = −1.25 D - Q11. A hypermetropic person can see objects clearly only beyond 1 m. Draw a ray diagram to show the defect and find the power of the corrective lens needed to read a book at 25 cm.
Object at u = −0.25 m must form a virtual image at the near point, v = −1 m.
1/f = 1/v − 1/u = 1/(−1) − 1/(−0.25) = −1 + 4 = 3 ⇒ f = 1/3 m. Power = +3 D (converging lens). - Q12. Why does a normal eye not see objects placed closer than 25 cm distinctly?
Below 25 cm, the ciliary muscles cannot contract further to shorten the eye-lens focal length enough to focus the image sharply on the retina — the image would form behind the retina, causing blurred vision. 25 cm is the least distance at which the eye can still comfortably focus. - Q13. What happens to the image distance in the eye when we increase the distance of an object from the eye?
The image is always formed on the retina, so the image distance (eye-lens-to-retina distance) stays essentially constant; instead, the ciliary muscles relax, increasing the eye lens’s focal length as the object moves farther away. - Q14. Why do stars twinkle?
Starlight undergoes continuous refraction as it passes through the atmosphere, whose density (and hence refractive index) varies with altitude and keeps fluctuating. This causes the apparent position/brightness of the star to vary rapidly, seen as twinkling. - Q15. Explain why planets do not twinkle, while stars do.
Planets are much closer to Earth and appear as extended sources (a collection of many point sources). The twinkling effect of the many individual points averages/cancels out, so the net variation is negligible — planets appear to shine steadily. - Q16. Why does the Sun appear reddish early in the morning?
At sunrise, sunlight travels a much longer path through the atmosphere. Shorter wavelengths (blue/violet) are scattered away almost completely along this long path, while the longer-wavelength red light is scattered the least and reaches the eye directly, making the Sun appear reddish. - Q17. Why does the sky appear dark instead of blue to an astronaut?
At high altitudes there is little or no atmosphere to scatter sunlight. Since scattering is what makes the sky appear blue from the ground, the absence of a scattering medium means no light reaches the astronaut’s eyes from other directions, so the sky appears black even though the Sun is shining brightly.
FAQ
Q: Is this content updated for the 2026-27 NCERT edition?
A: Yes. This chapter is Chapter 10 in the current rationalised numbering (shifted from the earlier Chapter 11) — confirmed via cross-checking multiple independent current-session sources before writing.
Q: What is the single most important formula in this chapter?
A: Power of a lens, P = 1/f (f in metres, unit dioptre D) — concave lens power is negative, convex lens power is positive.
