IGCSE Physics / Light waves
Critical angle and total internal reflection
Edexcel International GCSE Physics (4PH1), chapter 12: Light waves, specification point 3.21, 3.22. Part of the full course.
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The critical angle is the angle of incidence at which the refracted ray travels along the boundary. Above this angle, light can undergo total internal reflection, which is useful for trapping light inside a material.