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.

 

Refraction

Refraction of light is the bending of light as it travels from one medium to another. You have seen this without realizing it when you look at an object under water in a pool while you are on the surface. Another example is when you look at a straw in a glass of water, the straw looks bent. However, it is refraction that makes the straw look bent.

When light travels from one medium to another (for example air into glass or plastic) the light will bend because of the difference in density. A material like glass or plastic will have more atoms per unit volume than air, so the light will bend as they hit the atoms. A material's ability to cause light to "bend" is defined by its index of refraction, n.

 

The equation explaining the bend is given by n1sinΘ1= n2sinΘ2  where n is the index of refraction of the material (either material 1 or material 2) and Θ is the angle of refraction measured from the normal. If the light comes in normal (i.e. perpendicular to the interface of the two materials), the light does not bend because Θ is 0 and sin Θ is 0.


 


Another consequence of light refraction is that the speed of light changes as it transfers from one medium to another. This is given by v1/n1=v2/n2 where v is the speed in medium. So the speed of light really is not a constant if we go from one medium to another. In fact, the speed of light you know and love is measured in a vacuum with n = 1. All other material (including air) has an index of refraction greater than 1, which means that the speed of light is slower in every material in the universe, since by definition, a vacuum is a lack of material.


Refraction is what causes lenses to work and why telescopes with lenses are called refractors. The lenses at the objective end of the telescope and at the eyepiece end bend the light towards your eye (or the detector).


One last consequence of refraction is that the bending is also dependent on the wavelength of the light. The longer the wavelength, the less the light bends since the longer wavelengths have an "easier" time avoiding the atoms in the material. This is how a prism works. Red light is bent less than blue light and when you use a prism on white light (defined as the combination of all colors of light), you get a rainbow of color. This is also how astronomers get spectra of stars and galaxies by using prisms on their light and measuring the emission and absorption lines in the light profile.