Critical Angle

Critical angle is the angle at which the light ray travelling form a denser medium to a rarer medium grazes along the surface rather than escaping directly. The angle of refraction in 90° for the angle of incidence equal to critical angle. A ray of light which is incident of the boundary separating the two optical mediums is incident at angle greater than critical angle will get reflected. This phenomenon is called Total Internal Reflection.


The angle of incidence is measured with respect to the normal at the refractive boundary (see diagram illustrating Snell's law). Consider a light ray passing from glass into air. The light emanating from the interface is bent towards the glass. When the incident angle is increased sufficiently, the transmitted angle (in air) reaches 90 degrees. It is at this point no light is transmitted into air. The critical angle  is given by Snell's law,

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Rearranging Snell's Law, we get incidence

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To find the critical angle, we find the value for  when 90° and thus . The resulting value of  is equal to the critical angle .

Now, we can solve for , and we get the equation for the critical angle:

If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence. If for example, visible light were traveling through acrylic glass (with an index of refraction of 1.50) into air (with an index of refraction of 1.00), the calculation would give the critical angle for light from acrylic into air, which is

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Light incident on the border with an angle less than 41.8° would be partially transmitted, while light incident on the border at larger angles with respect to normal would be totally internally reflected.

If the fraction  is greater than 1, then arcsine is not defined—meaning that total internal reflection does not occur even at very shallow or grazing incident angles.

So the critical angle is only defined when  is less than 1.