Concave mirrors

Let's make a concave mirror. We start off with a complete circle. It seems quite obvious at the moment that the middle of the circle is the centre of curvature.



















Now let's mark out a section of the circle and then cut it out. You can see that it still has a centre of curvature even when it's just a cut section.



















Now let's fire rays of light at it. Notice that they don't focus at the centre of curvature. That means that we also need to know about a second point - the focal point (or principal focus).

















Now take a look at a proper ray diagram:

• It has C for centre of curvature and F for principal focus
• The curved lens has been drawn flat for the purposes of the diagram

There are 3 possible rays you could draw, although only 2 appear on any diagram:

Ray 1: Horizontally from the tip of the object arrow to hit the mirror and bounce back through F.
Ray 2: Slants down straight through F to hit the mirror then bounces back horizontally.

Ray 3: Is a perfectly straight line going through the top of the image arrow and through point C.