# WANTED: Proof of formula!

1. Sep 28, 2004

### shashank010288

The formula for no. of images by two mirrors inclined at $angle is [ 360/$]-1
can anybody prove it?

2. Sep 28, 2004

### Gokul43201

Staff Emeritus
Consider an angle

$$360/(n+1) < \theta < 360/n$$

Find the positions of the images, starting from each of the pricipal images. When two images have the same position, stop.

I'll try this myself, when I find the time.

3. Sep 29, 2004

### shashank010288

I want a proof for general angle

4. Sep 29, 2004

### pervect

Staff Emeritus
Well, the first goal is to prove that the number of images is independent of the poistion of the observer, or perhaps you could specify the position of the observer.

5. Sep 29, 2004

### Gokul43201

Staff Emeritus
Any angle Ccan be shown to satisfy the above criteria for a suitable choice of n.

6. Sep 29, 2004

### Tide

If the inclination of the second mirror with respect to the first is $\beta$ then the angle of incidence is increased by $\beta / 2$ at each reflection.