First slide

Ray optics

Question

For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be

Difficult

A

1.67 cm
B

1.5 cm
C

5 cm
D

2.5 cm

Solution

Converging power of cornea $P_{c} = + 40 D$
Least converging power of eye lens, $P_{e} = + 20 D$
Power of the eye-lens, $P = P_{c} + P_{e} = 40 D + 20 D = 60 D$
Power of the eye lens $P = \frac{1}{F o c a l l e n g t h o f t h e e y e l e n s (f)}$
$f = \frac{1}{P} = \frac{1}{60 D} = \frac{1}{60} m = \frac{100}{60} c m = \frac{5}{3} c m$
Distance between the retina and cornea-eye lens
= Focal length of the eye lens $= \frac{5}{3} c m = 1.67 c m$

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