The Human Eye and the Colourful World — Class 10 Science
Science · Chapter 10
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Chapter summary
Key ideas & terms
- Accommodation
- The eye lens adjusting its focal length to focus near and far objects.
- Myopia / hypermetropia
- Short-sightedness / long-sightedness defects of vision.
- Dispersion
- Splitting of white light into its component colours.
- Scattering of light
- Redirection of light by particles, e.g. making the sky blue.
Extra questions & answers
Why is the sky blue on a clear day?
Air molecules scatter the shorter-wavelength blue light far more than red, so the scattered light reaching our eyes is predominantly blue.
Why does the sun appear reddish at sunrise and sunset?
Light travels through more atmosphere, scattering away most blue light and leaving the longer-wavelength red to reach us.
What is the power of accommodation of the eye?
The ability of the eye lens to change its focal length (by the action of the ciliary muscles) so as to focus on objects at different distances.
What is myopia, its cause and correction?
Myopia (short-sightedness) is the inability to see distant objects clearly because the image forms in front of the retina; it is corrected with a concave (diverging) lens of suitable power.
What is hypermetropia, its cause and correction?
Hypermetropia (long-sightedness) is the inability to see nearby objects clearly because the image forms behind the retina; it is corrected with a convex (converging) lens.
What is the least distance of distinct vision for a normal eye?
About 25 cm; this is the closest distance at which the eye can see an object clearly and comfortably.
Why does a glass prism split white light into colours?
Different colours of light bend by different amounts (they have different speeds in glass), so white light is dispersed into its spectrum — violet bends the most and red the least (refer to the prism diagram in your NCERT textbook).
Why do stars twinkle but planets do not?
Starlight is refracted continuously by the changing atmosphere, making stars appear to twinkle; planets are nearer and appear as a collection of point sources, so their light averages out and they do not twinkle.
Why is the sky blue on a clear day?
Air molecules scatter the shorter-wavelength blue light much more than red, so the scattered light reaching our eyes is mainly blue.
Why does the sun appear reddish at sunrise and sunset?
Near the horizon, sunlight travels through a greater thickness of atmosphere, which scatters away most of the blue light, leaving the longer-wavelength red light to reach us.
QUESTIONS
1. What is meant by power of accommodation of the eye?
The power of accommodation of the eye is its ability to adjust the focal length of the eye lens so that objects at different distances can be focused clearly on the retina.
2. A person with a myopic eye cannot see objects beyond
A concave lens of suitable power should be used. It diverges the incoming light rays so that the myopic eye can focus the image of a distant object on the retina.
3. What is the far point and near point of the human eye with normal vision?
For a normal human eye, the far point is at infinity, while the near point is approximately
4. 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 student is likely suffering from myopia, or near-sightedness, because distant objects such as the blackboard are not seen clearly. It can be corrected by using spectacles containing a concave lens of suitable power.
EXERCISES
1. The human eye can focus on 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.
The correct answer is (b) accommodation. Accommodation enables the eye lens to change its focal length and focus objects at different distances on the retina.
2. The human eye forms the image of an object at its
(a) cornea. (b) iris. (c) pupil. (d) retina.
The correct answer is (d) retina. The eye lens forms a real and inverted image of the object on the retina.
3. The least distance of distinct vision for a young adult with normal vision is about
(a)
The correct answer is (c)
4. 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.
The correct answer is (c) ciliary muscles. These muscles change the curvature and therefore the focal length of the eye lens.
5. A person needs a lens of power
The focal length is calculated using
For distant vision,
For near vision,
Therefore, the required focal lengths are (i) approximately
6. The far point of a myopic person is
A concave lens is required. For a distant object,
Using
we get
Therefore,
Thus, a concave lens of power
7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is
In the correction diagram, a convex lens is placed in front of the hypermetropic eye. It makes the rays from an object at the normal near point appear to come from the near point of the defective eye, allowing the eye lens to focus them on the retina.
Here,
Using the lens formula,
Thus,
Therefore,
A convex lens of power approximately
8. Why is a normal eye not able to see clearly the objects placed closer than
When an object is placed closer than
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
The image distance remains nearly constant because the distance between the eye lens and the retina does not change. As the object distance increases, the eye adjusts the focal length of its lens to keep the image focused on the retina.
10. Why do stars twinkle?
Stars twinkle because their light undergoes continuous atmospheric refraction while passing through layers of air whose refractive indices keep changing. Since a star behaves like a point source, these variations cause its apparent position and the amount of light entering the eye to fluctuate, making it appear alternately brighter and fainter.
11. Explain why the planets do not twinkle.
Planets are much closer to Earth than stars and appear as extended sources of light. Light reaches us from many points on a planet's visible surface. The variations in brightness caused by atmospheric refraction at different points average out, so planets generally do not twinkle.
12. Why does the sky appear dark instead of blue to an astronaut?
The sky appears dark to an astronaut because space has no substantial atmosphere to scatter sunlight. Without atmospheric particles scattering blue light into the astronaut's eyes, the surrounding sky appears dark.