Reflection

The law of reflection is one of the simplest equations in all of physics:

$$\theta_r=\theta_i$$
That is, a ray which hits the interface is reflected at the same angle from the normal, but on the other side of the normal.

When the surface is particularly smooth, like a mirror or the surface of a calm pond, the normal is easy to determine, and one can see reflected images in the surface. This is called specular reflection. When parallel rays from a distant light source hit a very flat surface, they remain parallel afterwards, and so the eye looking at the surface is fooled into thinking that the light source is actually behind the surface.

Most surfaces, however, are not flat, but bumpy. In that case, the normals will point in all different directions, so that even parallel rays will be reflected in all different directions. This is called diffuse reflection, and it is how we see most objects. For example, the sun emits light in all directions because it is a light source. Some of that light is diffusely reflected from the surface of the moon, which is scattered in all directions in the same way: thus the moon appears to be a light source as well.