Cambridge O Level Physics · Syllabus 5054 · Waves
Critical Angle
What is Critical Angle?
The critical angle is the angle of incidence, measured in the optically denser medium, for which the angle of refraction in the less dense medium is ninety degrees, so the refracted ray travels along the boundary.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Critical Angle in context
Light changes speed when it crosses from one transparent medium into another, and if it meets the boundary at an angle to the normal it also changes direction. Entering a medium in which light travels more slowly, the ray bends towards the normal; entering one in which it travels faster, it bends away from the normal. A ray along the normal changes speed but not direction. The refractive index \(n = \dfrac{\sin i}{\sin r}\) measures how strongly a medium refracts light, and the critical angle \(c\), given by \(n = \dfrac{1}{\sin c}\), is the angle of incidence in the denser medium that produces a refracted ray along the boundary.
Common mistakes with Critical Angle
- 11. “The critical angle works in either travel direction.” Why wrongLight entering a denser medium bends towards the normal, so the angle inside can never reach 90°; there is nothing for a critical angle to mark. CorrectThe critical angle is defined for light travelling from higher refractive index to lower. Say this“The critical angle is the angle of incidence in the denser medium for which the angle of refraction is 90°.” CheckDoes a ray going from air into glass have a critical angle? (No.)
- 12. “Total internal reflection occurs at the critical angle.” Why wrongAt exactly \(c\) a refracted ray still exists — it travels along the boundary at 90° to the normal. CorrectTotal internal reflection needs the angle of incidence to be strictly greater than \(c\). Say this“Total internal reflection occurs when the angle of incidence is greater than the critical angle.” CheckWater has \(c = 49^\circ\). A ray inside the water strikes the surface at 49°. What happens?