Question

The focal length of a normal eye-lens is about:
$
  (a){\text{ 1mm}} \\
  (b){\text{ 2cm}} \\
  (c){\text{ 25cm}} \\
  (d){\text{ 1m}} \\
$

Answer 1

Hint: In this question use the concept that focal length is nothing but the distance between the eye lens of human beings and the retina present inside the eyes. This will help get the right default value of focal length of the human eye.

Complete Step-by-Step solution:

Consider an eye lens as shown in the figure and the retina of the eye acts as a screen on which the real image is formed.
So the focal length is the distance between the eye lens and the retina.
So when as a source of light coming from the infinity it passes through the eye lens and converges it on the retina as shown in the figure.
So for a normal eye lens the focal length is about 2 cm or 20 mm (when eye is in relaxed position).
So this is the required answer.
Hence option (B) is the correct answer.

Note – For a normal eye, when the eye muscles are relaxed, the focal length f of the lens L is slightly less than its diameter, which is D = 2.4 cm. An idealized eye will focus an object at infinity on the retina which is located at distance, D, behind the lens of the eye. We can say that focal length varies from person to person, that is why people tend to have hypermetropia or myopia.