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[This is a stub article for reference in physics, optics, and photography posts. It will be updated and expanded.]

Specular Reflection

The quality of reflected light depends mostly on the interface in which the light is interacting. Consider first an ideal, perfectly smooth, entirely reflective surface with no transmission or diffusion. The incoming angle is called the angle of incidence, \theta_i and the angle leaving the surface is the angle of reflection, \theta_r. When the angle of incidence is equal to the angle of reflection, \theta_i = \theta_r we say that reflection is a perfect specular reflection or regular reflection.

2-Dimensional diagram depicting ideal specular reflection. With specular reflection, the angle of incidence is equal to the angle of reflection.

The relation that the angle of incidence is equal to the angle of reflection combined with the observation that the normal direction, incident, and reflected rays lie in the same plane is known as the law of reflection.

Conditions for Specular Reflection

Specular reflection requires a high degree of surface flatness relative to the “object” that is being reflected. For day-to-day objects, like a racquetball, this means a smooth surface compared to the racquetball. The ball striking a surface like plexiglass will be nearly specularly reflected. If it strikes an irregular surface like a bumpy dirt road, it is very unlikely that \theta_i = \theta_r and you will get something called diffuse reflection where it may bounce in any direction.

What constitutes “flat” for light is a bit more nuanced since the reflection of light is an electromagnetic interaction between light and the reflecting material. I’ll go into more detail soon.

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