Physics Question on Ray optics and optical instruments

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 -

Answer 1

Solution

For a normal eye, rays coming from infinity should go the retina without effort when we look at infinity, lens offers minimum power and hence combination gives 40D + 20D = 60D.
Distance between the retina and the cornea eye has must be equal to focal length.
$f = \frac{1}{60} m = 1.67 cm$