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What is the power of accommodation of the human eye?

Class 10 ScienceNCERT chapter: The Human Eye and the Colourful Worldइसे हिंदी में पढ़ें

Exam-ready answer

The power of accommodation of the eye is the ability of the eye to change the focal length of its lens to focus on objects at different distances. The ciliary muscles adjust the lens curvature. The near point is 25 cm and the far point is infinity for a normal eye.

Explanation

The human eye has a convex lens made of flexible tissue. To see distant objects clearly, the ciliary muscles relax, the lens becomes thinner, and its focal length increases. To see nearby objects, the ciliary muscles contract, the lens becomes thicker, and its focal length decreases. This automatic adjustment is called accommodation.

The power of accommodation is the maximum variation in the power of the eye lens. It is measured in dioptres (D). For a normal eye, the near point is about 25 cm and the far point is infinity. The ability to see objects clearly from infinity to 25 cm is due to this power.

The ciliary muscles control the shape of the lens. When they contract, the lens becomes more curved, increasing its converging power for near vision. When they relax, the lens flattens, decreasing its converging power for distant vision. This reflex action allows us to see objects clearly at varying distances.

Points to remember

  1. Power of accommodation: ability of the eye to adjust the focal length of its lens.
  2. Ciliary muscles change the curvature of the lens.
  3. Near point of a normal eye: 25 cm; far point: infinity.
  4. With age the lens stiffens and accommodation weakens (presbyopia), so the near point moves farther away.
  5. Range of vision: from infinity to 25 cm.

Exam tip

Write the definition clearly and mention the near point (25 cm) and far point (infinity). Draw a simple labelled diagram of the eye showing the ciliary muscles and lens.

Questions students also ask

What is the near point of a normal human eye?
The near point is the closest distance at which the eye can see an object clearly. For a normal eye, it is about 25 cm. It increases with age due to weakening of ciliary muscles.
What happens to the focal length of the eye lens when we look at a nearby object?
When we look at a nearby object, the ciliary muscles contract, the lens becomes thicker, and its focal length decreases so that the image forms on the retina.
What is the role of ciliary muscles in accommodation?
Ciliary muscles control the shape of the eye lens. Their contraction and relaxation change the lens curvature, thereby adjusting its focal length for clear vision at different distances.

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