Question

Focal length of eye lens is

• A. fixed
• B. variable
• C. sometimes fixed and sometimes variable
• D. cannot say

Answer 1

The correct option is B

variable

• Power of accommodation is made possible by the adjustment of the focal length of the eye lens by the action of ciliary muscles holding the lens.
  
• Power of the eye lens is given by the formula,
  Power, $P=\frac{1}{S\text{'}}-\frac{1}{S}$
• Here S is the position of the object, while S’ is the position of the retina in the eye (which is fixed at 2.5 cm from the eye-lens).
  When the ciliary muscles are relaxed, the eye lens becomes thin, the focal length increases, and the distant objects($s=\infty$) are clearly visible to the eyes.
  To see the nearby objects ($s<\infty$ or at 25cm) clearly, the ciliary muscles contract making the eye lens thicker. Thus, the focal length of the eye lens decreases and the nearby objects become visible to the eyes.
  Hence, the human eye lens is able to adjust its focal length to view both distant and nearby objects on the retina. Hence focal length for the human eye is variable and it is not fixed.