Critical angle
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In geometric optics, at a refractive boundary, the critical angle is the angle of incidence above which total internal reflection occurs.
The angle of incidence is measured with respect to the normal at the refractive boundary. It is given by:
- <math>\theta_c = \arcsin \left( \frac{n_2}{n_1} \right) </math>,
where θc is the critical angle, n2 is the refractive index of the less dense medium, and n1 is the refractive index of the denser medium. This equation is a simple application of Snell's law where the angle of refraction = 90°.
Note: Total internal reflection only occurs when the incident ray is in the denser medium.
If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.
For visible light travelling from glass into air (or vacuum), the critical angle is approximately 41°.
Source: from Federal Standard 1037C and from MIL-STD-188