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

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

(a) 2.5 cm

(b) 1.67 cm

(c) 1.5 cm

(d) 5 cm

Answer :  (b) 1.67 cm

Concept / Approach: Cornea and eye lens are two parts of the human eye which help for converging the light rays on the retina. So the power of cornea and eye lens to added to find the combined effect, as we do in combination of lens. Then we know $P=\frac{1}{f}$. See the diagam, f so found will be the approx. size of eye ball.

\( P_{\text{total}} = 40D + 20D \)

\( \frac{1}{f} =  60 \, m^{-1} \)

\( \frac{1}{f} = \frac{60}{1m} \)

\( \frac{1}{f} = \frac{60}{100 \, cm} \)

\( f = \tfrac{5}{3} \, cm = 1.67 \,cm\)