Question

The focal length of the eye lens increases when eye muscles

• A. are relaxed and the lens becomes thinner
• B. contract and lens becomes thicker
• C. are relaxed and the lens becomes thicker
• D. contract and lens becomes thinner

Answer 1

The correct option is A

are relaxed and the lens becomes thinner

The explanation for the correct answer:

Option (A): are relaxed and the lens becomes thinner

1. In the case of the human eye, the lens is held by the ciliary muscle which can contract and expand to adjust the focus.
2. In the case of a thin lens, the refracting area is more so the focal length of the eye increases.
3. When the ciliary muscle is in a relaxed position the lens remains thinner so focal length increases and the image is formed at the retina even if we increase the distance of the object.

The explanation for the incorrect answer:

Option (B): contract and lens becomes thicker

1. When the lens becomes thick the refracting area decreases so the point of convergence of the ray comes closer and focal length decreases.

Option (C): are relaxed and the lens becomes thicker

1. In the case of a thicker lens, the focal length decreases because the point of convergence comes closer.

Option (D): contract and lens become thinner

1. When the muscles contract the lens becomes more spherical and hence the focal length decreases.

So, Option (A) is correct. The focal length of the eye lens increases when eye muscles are relaxed and the lens becomes thinner