Reflecting Prisms

[island]

Each prism in the cartadioptric section of a lens structure refracts and reflects light. Light enters one prism face, bends, and then reflects from a second face. It bends again as it emerges from the prism at the third face.

The question is: Why does the light not emerge from the second prism face instead of reflecting from it?

[Ross]

cartadioptric

"catoptron" means mirror and "dioptron" means lens in Greek

[beachbum1616]

Does it have something to do with the angle with which it hits the second surface face of the prism?

[island]

Absolutely. It is the angle.

This is sometimes referred to a the critical angle of reflectance and this angle can be calculated from the refractive index of the glass.

For a Crown glass prism with a refractive index of 1.52 the critical angle is 41.1 degrees.

This critical angle was a consideration when designing the frames to hold the catodiptric prisms.

[beachbum1616]

[island]

To further clarify , confound or confuse , (circle one)

The Critical Angle relates to total internal reflection that occurs inside the prism where the light ray is traveling through the dense glass and approaching the boundary with the less dense air outside the prism. This is the behavior of light rays inside the prism only and not the behavior of rays of light entering the prism.

Reflection occurs when the angle of approach to the boundary wall is greater than the Critical Angle. Refraction occurs when the angle of approach is less than the critical angle. When the approach is at the Critical Angle the emerging light ray refracts at 90 degrees, follows the boundary between the glass and the air and the light beam emerges at a corner point of the prism.