Waves - including sound and light - can be reflected at the boundary between two different materials. The reflection of sound causes echoes.
The law of reflection states that:
angle of incidence = angle of reflection
For example, if a light ray hits a surface at 32°, it will be reflected at 32°.
The angles of incidence and reflection are measured between the light ray and the normal - an imaginary line at 90° to the surface. The diagrams show a water wave being reflected at a barrier, and a light ray being reflected at a plane mirror.
Reflection from a smooth, flat surface is called specular reflection. This is the type of reflection that happens with a flat mirror. The image in a mirror is:
In a virtual image, the rays appear to diverge from behind the mirror, so the image appears to come from behind the mirror.
If a surface is rough, diffuse reflection happens. Instead of forming an image, the reflected light is scattered in all directions. This may cause a distorted image of the object, as occurs with rippling water, or no image at all. Each individual reflection still obeys the law of reflection, but the different parts of the rough surface are at different angles.