Question

For a normal eye, the cornea of eye provides as converging power of 40 D and the lens 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:

• A. 1.5 cm
• B. 5 cm
• C. 2.5 cm
• D. 1.67 cm

Answer 1

The correct option is D 1.67 cm

Given that,

Power $P_1=40D$

Power $P_2=20D$

Total power of the combination

$P=P_1+P_2$

$P=40+20$

$P=60D$

Now, focal length of the combination

$f=\frac{1}{60}\times 100$

$f=\frac{5}{3}cm$

Now, for minimum converging state of the eye lens object is at infinity

$u=-\infty$

$f=\frac{5}{3}$

Now, from lens equation

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{3}{5}=\frac{1}{v}-\frac{1}{\infty }$

$\frac{1}{v}=\frac{3}{5}$

$v=\frac{5}{3}cm$

So, the distance between retina and cornea – lens system $1.67cm$