How to Work out the Critical Angle of a Substance

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posted on 13 years ago — updated on 1 second ago

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The critical angle of a substance when passing a light ray from that substance to vacuum is very important as you must use it to determine whether total internal reflection will occur. It is also important for your physics test. Find out how to work out the critical angle!

Steps

Make sure you know the Snell's Law. The general form of Snell's Law is nxsinθx=nvacuumsinθvacuum, where n is the refractive index and θ is the angle of incidence or refraction.

Find out the refractive index of the substance (or nx).

You can now do the substitution. Let's say you are using glass with a refractive index of 1.50. Therefore, the critical angle is the angle which, when it is the angle of incidence, produces an emergent ray of 90°. nglasssinθglass=nvacuumsinθvacuumWork out the Critical Angle of a Substance Step 3Bullet1.jpg 1.50sinθglass=1sin90°Work out the Critical Angle of a Substance Step 3Bullet2.jpg sinθglass=2/3Work out the Critical Angle of a Substance Step 3Bullet3.jpg θglass=41.8° (correct to 3 significant figures)