Question

Explain how an eye lens changes its focal length. How does it help in seeing near and distant objects?

Answer 1

Definition of focal length

Focal length is the distance at which a collimated ray converges from infinity to a point on the optical axis.

Explanation

1. The ability of the eye lens to adjust the focal length, by stretching or relaxing the ciliary muscles is known as accommodation.
2. When the ciliary muscles relax, the lens becomes thinner. This will increase the focal length. This allows you to clearly see objects in the distance.
3. Given below is a diagram showing relaxed ciliary muscles making the eye lens thinner. Thereby allowing us to view far objects

1. . When you look at an object near your eyes, the ciliary muscle contracts. This increases the curvature of the crystalline lens of the eye.
2. The crystalline lens of the eye becomes thicker. The focal length of the eye lens will be shorter. This allows you to clearly see nearby objects.
3. Given below is the diagram of the human eye when the ciliary muscles contract making the lens thicker and decreasing the focal length of the crystalline lens