Reflection of Light from a surface

Incoming light interacts with a surface and may be absorbed, refelected, and/or transmitted. Materials have a reflectance spectrum which is a function of the angle of incidence of the incoming light. The color of an object is a function of the color spectrum of the incident light and the reflectance spectrum of the object surface. For example, if an object absorbed all the light except for red then the object would appear to be red in a white light. So the reflectance spectrum of a "red" object would look like this:


The reflectance spectrum of a real surface is generally more complex. For example, here is the reflectance spectrum of copper at normal incidence.

Image Reference

The reflectance spectrum is frequently a function of the incident angle. For example, here is how the reflectance spectrum of copper changes with the angle of incidence.

Image Reference

The reflected spectrum is the incoming light spectrum times the reflectance spectrum of the surface. For example the above "red" object illuminated with perfect white light would appear red:

A more complex example is copper. The top curve is the spectrum of the light source. It is multiplied at each wavelength by the reflectance spectrum of copper (as shown at the left) to produce the reflected spectrum.

Image Reference Image Reference

Physics of Reflection

When a photon strikes a molecule it generally imparts energy as heat and the photon is absorbed. For some photons their energy is equal to the resonance energy of the molecule. When this happens the photon is absorbed and the molecule is put into an excited state. The molecule relaxes from the excited state back to the ground state by emitting a photon with energy equal to that of the original photon so the photon appears to have been reflected (but is actually absorbed then emitted). The emitted photon can go in any direction so this is the mechanism for diffuse reflection. For specular reflection the photons actually "bounce" off of the surface and are not absorbed and then re-radiated.

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Last changed March 13, 1999, G. Scott Owen,