Focal Length of a Plano-convex Lens

[scar_face]

The given problem:

What is the focal length of a Plano-convex lens, assuming parallel rays of light from s=∞ is traveling from the flat end of the lens. The lens is a glass hemisphere of radius R. Additionally the index of refraction of glass is higher than that of air.

Homework Equations

General lens maker equation:
ng/s +nair/s' = (ng + nair)/R
==> assuming s=∞; s'=R/(ng-nair)

The Attempt at a Solution

I do not know if the distance to the focal point (i.e., the focal length s', as s=∞) is the distance starting FROM the point where the light hits the lens, or where the light exits the lens?

 

[HallsofIvy]

The given problem:

What is the focal length of a Plano-convex lens, assuming parallel rays of light from s=∞ is traveling from the flat end of the lens. The lens is a glass hemisphere of radius R. Additionally the index of refraction of glass is higher than that of air.

Homework Equations

General lens maker equation:
ng/s +nair/s' = (ng + nair)/R
==> assuming s=∞; s'=R/(ng-nair)

The Attempt at a Solution

I do not know if the distance to the focal point (i.e., the focal length s', as s=∞) is the distance starting FROM the point where the light hits the lens, or where the light exits the lens?

The focal length is not either of those. It is the distance from the focus to the point where the line through the focus, parallel to the axis of the lens, passes through the curving face. The focal length has nothing to do with the direction from which the light is coming, or, indeed, if there is any light at all!

 

[scar_face]

So, what does s' represent in this particular case?