Reflection

The reflection of light is just the bouncing of a light wave or a photon (however you want to look at it) from a surface. The reflection of light off a flat surface is pretty simple to understand.


Light comes in at an angle and is reflected off the surface at the same angle. Using physics terms, the angle of incidence is equal to the angle of reflection and are measured from the normal to the surface. Recall that the normal to a surface is an imaginary line that is perpendicular to that surface, in this case, a perpendicular to the point of reflection.

In cases of spherical mirrors, the reflection gets a little more complicated. If you have light coming into the mirror in parallel beams or rays from the same source, the light will not be reflected to the same point. This is called spherical aberration.

But the objective mirror of a reflector is a curved mirror! And they don't suffer from spherical aberration. How does this work?


Instead of using spherical mirrors, reflectors use parabolic mirrors. A parabola is a shape that has a focal point. This way, when a mirror has a parabolic shape, parallel light beams will be reflected to the same point in front of the mirror. A reflector then will use a secondary mirror to reflect the light off to the side (Newtonian reflector) or back down the end (Cassegrain reflector).

Parabolic mirrors are not as extremely curved as the one shown above, but I've exaggerated the curvature so that it can be seen how all incident light parallel to each other (i.e. coming from the same source) will reflect off the mirror and reach a common focal point on the reflected path.

See my post on telescopes for more about the different types of telescopes.