# Question

A method of determining the focal length of a diverging lens is called auto collimation. As Fig. 23.30 shows, first a sharp image of a light source is projected onto a screen by a converging lens. Second, the screen is replaced with a plane mirror. Third, a diverging lens is placed between the converging lens and the mirror. Light will then be reflected by the mirror back through the compound-lens system, and an image will be formed on a screen near the light source. This image is sharpened by adjusting the distance between the diverging lens and the mirror. The distance at which the image is clearest is equal to the focal length of the lens. Explain why this method works.

## Answer to relevant Questions

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