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I'll start (from my Physics homework):

A woman looks at herself in a magnifying converging mirror whose focal length is 20cm. If her face is 10cm from the mirror,

a. At what distance from the mirror is her image?

b. What is the magnification of her face?

Harder than it looks...

a. 10cm

b. Read the info on the back of the mirror, if it says 2x it's because it's two times bigger. If it says 10x it's ten times bigger, and so on...

Easy, huh?

Um, no. That's not how mirrors work.

Her image is 20 cm behind the surface of the mirror, magnified 200%, I believe.

When dealing with concave mirrors, the inverse of the focal length is equal to the sum of the inverse distance between the object and the mirror and the inverse distance between the image and the mirror. Solving 1/f = 1/d

_{0} + 1/d

_{i} for d

_{i} we get -20, which is 20 cm behind the mirror.

The magnification is equal to the negation of the distance between the object and the mirror over the distance between the image and the mirror. Solving M = -(d

_{i}/d

_{o}) for M, we get 2.

@Bulle: I know that one too, or at least a variation of it.