Specular reflection

Diagram of specular reflection

Reflections on water are an example of specular reflection.

Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light (or other waves) from a single incoming direction is reflected onto a single outgoing direction. Such behaviour is described by the law of reflection, which states that the direction of outgoing reflected light and the direction of incoming light make the same angle with respect to the surface normal; this is commonly stated as $\theta _{i}=\theta _{r}$.

This is in contrast to diffuse reflection, where incoming light is reflected in all directions equally. The most familiar example of the distinction between specular and diffuse reflection would be matte and glossy paints. Matte paints have a higher proportion of diffuse reflection, while glossy paints have a greater proportion of specular reflection.

Specular reflection and diffuse reflection are simply approximations. In reality, surfaces exhibit a continuum of modes of reflection between these two. Also, not all energy of the wave may be reflected by the surface and some energy may penetrate into the second medium behind the surface. The greater the difference in the density of the two media on either side of the surface and the greater the angle $\theta _{i}$ the greater part of the energy is reflected. When $\theta _{i}$ is greater than a given threshold then the entire energy of the wave is reflected from the surface and nothing penetrates through it. This phenomenon is known as full specular reflection. At home it can be observed and demonstrated with many different surfaces (e.g. glass, porcelain, plastic, etc.)

Specular reflection is very important for making good scratch holograms, which are optically similar to Benton Rainbow Holograms (AKA: "White Light Holograms"); see also SPIE article and the FAQ, and the main Wikipedia Holography entry.