Lenses Question

1. Jul 30, 2011

fromthepast

I know that for concave lenses, the focal length is one half the length of the radius of the circle.

Is this also true for convex lenses?

Thanks

2. Jul 30, 2011

xts

That's not true. Similar rule applies to mirrors, but not to lenses.

3. Jul 30, 2011

Staff: Mentor

4. Jul 30, 2011

fromthepast

What I meant to say was:

"I know that for concave MIRRORS, the focal length is one half the length of the radius of the circle.

Is this also true for convex MIRRORS?"

5. Jul 30, 2011

xts

Yes, it is.
Just draw yourself a picture how the light reflects from the spherical surface...

6. Aug 1, 2011

JeffKoch

The number is the same for the same radius, but the sign flips - a convex mirror has a negative focal length, with the usual sign conventions. This makes sense if you think about it - a collimated object beam has the object at infinity, but the reflected beam diverges from a virtual source with a negative image distance. Therefore the focal length must be negative.