Question

The focal length of a normal eye-lens is about

Options

• 1 mm

•  2 cm

•  25 cm

• 1m

Answer 1

 2 cm

Given:
Near point of the human eye, u = -25 cm
Distance between the retina and the eye lens, v = 2 cm (approximately)
Thus, we have the focal length, f.

`1/f=1/v-1/u`

`=>1/f ≅1/2 - 1/-25`

`=>1/f ≅27/50`

⇒ f ≅ 2 cm