Question

The optical prescription for a pair of spectacles is:

Right eye: −3.50 D 

Left eye: −4.00 D

1. Are these lenses thinner at the middle or at the edges?
2. Which lens has a greater focal length?
3. Which is the weaker eye?

Answer 1

1. These lenses are thinner in the middle because they are concave lenses. Further, concave lenses have negative power.
2. A lens of lower power has a greater focal length. So, −3.50 D lens has a greater focal length
3. The left eye is the weaker one because it needs a lens of greater power for its correction.