Re: what is the min. angle of incidence for a prism to refract a ray of light?

A ray of light will fail to pass through if it is internally reflected within the prism. Below is a figure shows a light ray (blue line) passing through an equilateral prism. As angle A is reduced there will reach a point where angle D reaches 90 degrees, which is the condition for total internal reflection. Below the figure is the derivation of this critical angle.

For a typical value for the refractive index of glass of n=1.5, the formula gives a critical angle of A=28 degrees.