Question

What would be the focal length of a normal human eye lens for an object lying at infinity ?

O)2.5 cm

O)3.3 cm

O)1.7 cm

O)0.9 cm​

Answer 1

The normal focal length of human eye (for an object at infinity) is 1.7 cm.

• First of all, the total converging power of normal eye is +63D, contributed by cornea (+43D) and lens (+23D).

So, the focal length of eye comes as:

$\rm \: P = \dfrac{100}{f \: (in \:centimetres) }$

$\rm \implies\: f= \dfrac{100}{ P}$

$\rm \implies\: f= \dfrac{100}{63}$

$\rm \implies\: f = 1.58 \: cm$

$\rm \implies\: f \approx \: 1.7\: cm$

So, net focal length of normal eye is 1.7 cm.

• But, please remember that the net converging power and the focal length of the eye changes when the object distance from the eye changes.

• For example: when the object comes closer to the higher the converging power of the eye increases and hence the focal length decreases.

Hope It Helps.