I need to figure out how to find the focal length of a makeup mirror. How can i do this?

1. shine a light at the mirror


2. go to a wall and move the mirror closer and farther away from the wall til the light is focused into a dot


3. measure the length from the mirror to the wall


It's that simple


PS please pick me as best answerI need to figure out how to find the focal length of a makeup mirror. How can i do this?
The light has to be as far from the mirror as possible. So use the Sun , focus its image to form the smallest possible point ( be careful this might start burning your screen)


The distance form the point image of the centre of the mirror is the focal length of the mirror.





1/f = 1/u + 1/v





f is the distance you want, u is the object distance which is effectively infinity if you use the Sun as your object, and v is the distance of the image from the mirror.





If u is very large, 1/u becomes zero





so 1/f = 0 + 1/v which can only be true if f = v





Easy , eh?