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The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to

- presbyopia
- accommodation
- near-sightedness
- far-sightedness

Answer :

Answer:

**(b)**accommodation

- The human eye forms the image of an object at its cornea iris pupil retina
- The least distance of distinct vision for a young adult with normal vision is about 25m 2.5cm 25cm 2.5m
- The change in focal length of an eye lens is caused by the action of the pupil retina ciliary muscles iris
- A 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 ?
- The 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?
- Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropia 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.
- Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
- Why do stars twinkle?
- Explain why the planets do not twinkle.
- Why does the Sun appear reddish early in the morning?
- Why does the sky appear dark instead of blue to an astronaut?

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