Question

Why is the focal length of a thick lens less and a thin lens more?

Answer 1

Assuming lens are made of the same material, the thicker one will be able to bend the light more as the light travels through it for a longer period of time. As we know, greater the bending lesser the focal length, so thin one will have more focal length.

The focal length depends on how sharply the light rays are bent when they pass through the glass. If the glass bends the rays sharply, they will converge at a short distance from the lens, so the focal length will be small. If the glass bends the rays only a little, they'll go for a longer distance before they converge.