Finding focal length of a convex lens with a laser

[wg1337]

Homework Statement

I have an actually interesting practical homework. I need to calculate the optical strength (hope I translated it right, D=1/F), to do I need to find the lens focal length. I need to find my self a concave and a convex lens. I don't have a concave lens (I think), but I do have a 200mw laser with and convex lens, so one part I can do. I don't know how could I find the focal length for concave and I don't seem to know where to get such a lens.

Homework Equations

For convex lens I could simply use D=1/F, so if I put my laser bit away from a sheet of paper and if the paper burns (my laser can do it) then it is the focal length.
For concave lens I could use this: D=1/f + 1/d, where f is length to screen and d is length to object and magnification is f/d.

The Attempt at a Solution

I think my way with finding focal length with laser is enough good, but maybe there is a better way.
But I don't like my idea about how to get focal length for concave lens:
I use the laser and let it go trough this lens, I move the lens/laser away to find when the laser dot is, for example, 10 times bigger. But don't know really how would it work.
Any better ideas?
Where could I find (in some device maybe) a concave lens?

 

[Delphi51]

You might try an optometrist's office for a concave lens. Often they have a large box of donated old glasses for third world countries and they likely would be glad to let you take a pair.

Maybe you could observe the reflection of the laser beam from the curved surface of a concave lens. The distance from the lens to the bright spot would give the focal length of the lens if the other side of the lens is flat.

We used to experiment with a candle and lens mounted on a meter stick. If you can accurately observe the location of the virtual image you can calculate the focal length.