Caption: A diagram illustrating the dispersion of a light ray by a prism and plate glass.

Features:

1. Snell's law (AKA the law of refraction) is

     n_1*sin(θ_1) = n_2*sin(θ_2)  , 

   where the n_i are the refractive indexes n = c/v_i (where c is the vacuum light speed c = 2.99792458*10**8 m/s and v_i is the light speed in medium i: c ≥ v_i) and the θ_i are the angles of light ray from the normal (i.e., the perpendicular) to the medium interface.

   Note

   sin(θ_2) = (n_1/n_2)*sin(θ_1)  , 

   and so θ_2 subceeds/exceeds θ_1 if n_2 is greater/lesser than n_1.

   Most common transparent solids and liquids have n_i greater than n_air, and so light rays bend toward/away the normal when entering/leaving these common materials.

2. Refractive indexes are generally wavelength dependent. In particular, common transparent solids and liquids have refractive indexes that are noticeably wavelength dependent.

3. Because of a prism's oblique medium interfaces, a polychromatic light ray (and therefore a light ray of mixed different wavelength and frequency) undergoes dispersion into continuum colors at a continuum of angles.

   Note the oblique medium interfaces cause the dispersion to increasing going through the second medium interface rather than cancel.

   Thus, dispersion creates a spectrum.

4. At long range from the prism, the dispersion of multiple incident light rays making up a light beam does NOT result in a remixing of the different colors into the original polychromatic light. The angular spreading out wins out over the remixing effect of closely packed incident light rays.

5. The situation for plate glass is DIFFERENT. There is NO spreading out in angle because the spreading out caused by entering at the first interface is canceled by exiting at the second interface.

   So any dispersion from oblique incidence is canceled by a remixing of the colors.

   Note the diagram is WRONG in showing refraction for light rays incident along the normal. Light rays incident along the normal do NOT refract.

6. If the interfaces of plate glass are NOT exactly parallel, some noticeable dispersion can occur. Dispersion can be seen near the edges of eyeglasses for example.

Credit/Permission: © David Jeffery, 2003 / Own work.
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