Prisms and Dispersion

Light passing through a prism refracts at both faces and emerges deviated toward the base. The total turn is the angle of deviation $\delta$.

• It depends on the angle of incidence, the prism angle $A$, and the refractive index.
• The deviation is a minimum at one symmetric angle (the angle of minimum deviation, $\delta_m$), where $n = \dfrac{\sin\!\left(\frac{A + \delta_m}{2}\right)}{\sin\!\left(\frac{A}{2}\right)}$.

The bending is refraction (a speed change at each face), not reflection.

Dispersion is the splitting of white light into its component colours (VIBGYOR) by a prism.

• A medium's refractive index is highest for violet, lowest for red — so violet's speed in glass is the lowest and red's the highest.
• Hence violet deviates the most, red the least.
• The cause is refraction (wavelength-dependent index), not reflection.
• Order of increasing deviation: Red < Orange < Yellow < Green < Blue < Violet.

A rainbow forms by dispersion in water droplets:

• Primary rainbow: sunlight undergoes refraction → one internal reflection → refraction inside each drop. It is the inner (lower) bow, brighter, with red on the outside.
• Secondary rainbow: involves two internal reflections, is fainter, sits outside the primary, with colours reversed.

So the rainbow is fundamentally a dispersion phenomenon (refraction + internal reflection combined), not simple reflection.