Chapter 10: The Human Eye and the Colourful World Exercise & In-Text Questions & Answers
1What is meant by power of accommodation of the eye?
Answer:
The ability of the eye lens to adjust its focal length by changing its curvature with the help of ciliary muscles is called the power of accommodation. This enables the eye to focus on both nearby and distant objects clearly. (Section 10.1.1)
2A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Answer:
A concave lens of suitable power should be used to correct myopia. Since the far point is 1.2 m (instead of infinity), the lens must diverge light rays from infinity so that they appear to come from 1.2 m. (Section 10.2 – Myopia)
3What is the far point and near point of the human eye with normal vision?
Answer:
For a normal human eye:
• Far point = Infinity
• Near point = 25 cm (least distance of distinct vision)
(Section 10.1.1)
4A 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?
Answer:
The child is likely suffering from myopia (near-sightedness). This defect can be corrected by using spectacles with a concave lens of appropriate power. (Section 10.2 – Myopia)
5The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to
Answer: (b) accommodation
(Section 10.1.1)
6The human eye forms the image of an object at its
Answer: (d) retina
The retina is the light-sensitive screen where the inverted real image is formed. (Section 10.1)
7The least distance of distinct vision for a young adult with normal vision is about
Answer: (c) 25 cm
(Section 10.1.1)
8The change in focal length of an eye lens is caused by the action of the
Answer: (c) ciliary muscles
(Section 10.1.1)
9A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer:
Focal length f (in metres) = 1 / Power (in dioptres)
(i) For distant vision: f = 1 / (–5.5) ≈ –0.182 m = –18.2 cm
(ii) For near vision: f = 1 / (+1.5) ≈ +0.667 m = +66.7 cm
(Based on lens power formula from optics, Section 10.2)
10The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer:
For myopia, a concave lens is used.
Far point = 80 cm = 0.8 m ⇒ focal length f = –0.8 m
Power = 1 / f = 1 / (–0.8) = –1.25 D
So, a concave lens of power –1.25 dioptres is required. (Section 10.2 – Myopia)
11Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Answer:
To see an object at 25 cm clearly, the lens must form a virtual image at 1 m (100 cm) for the defective eye.
Using lens formula: 1/f = 1/v – 1/u u = –25 cm, v = –100 cm
1/f = (–1/100) – (–1/25) = –0.01 + 0.04 = +0.03 cm⁻¹ = +3 m⁻¹
⇒ Power = +3 D
A convex lens of +3 dioptres is required.
(Section 10.2 – Hypermetropia)
Figure: Correction of hypermetropia using a convex lens
12Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer:
The ciliary muscles cannot make the eye lens thicker beyond a certain limit. Hence, the focal length cannot be decreased further to focus objects closer than 25 cm, causing blurred vision or eye strain. (Section 10.1.1)
13What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer:
The image distance in the eye remains constant because the image is always formed on the retina. The eye adjusts the focal length of the lens (via accommodation) to keep the image focused on the retina regardless of object distance. (Section 10.1)
14Why do stars twinkle?
Answer:
Stars twinkle due to atmospheric refraction. Starlight undergoes continuous refraction in layers of air with varying refractive indices, causing the apparent position and brightness of the star to fluctuate. (Section 10.5)
15Explain why the planets do not twinkle.
Answer:
Planets are closer and appear as extended sources (not point-sized). The fluctuations in light from different points average out, nullifying the twinkling effect. (Section 10.5)
16Why does the sky appear dark instead of blue to an astronaut?
Answer:
In space, there is no atmosphere to scatter sunlight. Since scattering (especially of blue light) does not occur, the sky appears dark. (Section 10.6.2)
T1Summary of Vision Defects and Corrections
Defect
Cause
Correction
Myopia (Near-sightedness)
Excessive curvature of lens or elongated eyeball
Concave lens
Hypermetropia (Far-sightedness)
Long focal length or small eyeball
Convex lens
Presbyopia
Weakening of ciliary muscles with age
Bifocal lenses (concave + convex)
(Section 10.2)
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