Optics Challenge #3 — Total Internal Reflection

The critical angle
in glass.

Optics Hard

Problem

Light travels inside a block of glass of refractive index 1.5 toward a boundary with air. Calculate the critical angle beyond which the light is totally internally reflected.

Refractive index\( n = 1.5 \)
Outside mediumair, \( n = 1 \)
Answer indegrees

At the critical angle, the refracted ray grazes along the surface at exactly 90°.

Hint: put the 90° refraction into Snell’s law: \( n \sin c = 1 \cdot \sin 90^{\circ} \), so \( \sin c = \tfrac{1}{n} \). Then undo the sine.

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The critical anglein glass.

Problem