2.8C: Power of a Mirror
( \newcommand{\kernel}{\mathrm{null}\,}\)
In Figure II.13 shows a reflecting surface of radius of curvature r submerged in a medium of index n. I show a real object at O, a virtual image at I and the centre of curvature at C. We see that h=αp=βq=γr. By Euclid, θ=α+γ and 2θ=α+β. Remember again that all angles are supposed to be small (even β!), in spite of the drawing. From these we obtain
1q=1p+2r.
On multiplying this by −n, we find that the power is −2n/r. Again the reader should try this for other situations, such a concave mirror, or a real image, and so on. The same result will always be obtained.