Cambridge O Level Physics · Syllabus 5054 · Waves
Refraction
What is Refraction?
Refraction is the change in direction of a wave caused by a change in its speed when it passes from one medium into another; frequency stays the same while speed and wavelength both change.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Common mistakes with Refraction
- 5. “Refraction occurs because the amplitude changes.” Why wrongAmplitude controls how much energy the wave carries. It has no influence on direction; a very quiet and a very loud sound refract identically. CorrectRefraction is caused by a change of speed. A wavefront arriving obliquely pivots because one end slows before the other. Say this“Refraction occurs because the wave changes speed at the boundary.” CheckState the one quantity whose change causes refraction.
- 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.)
Questions students ask about Refraction
Why are angles in reflection and refraction always measured from the normal, not the surface?
Every optical law in this chapter — the law of reflection, \(n = \sin i/\sin r\), and \(n = 1/\sin c\) — is defined with respect to the normal, the line drawn at 90° to the surface at the point where the ray strikes it. Measuring from the surface instead gives the complement of the angle, \(90^\circ - i\), which is not what the law describes. Always draw the dashed normal first.
Last reviewed Syllabus 2026-2028