| English | Critical angle |
|---|---|
| Bahasa Melayu | Sudut genting |
| 中文 | 临界角 |
Definition
The angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 90°.
In exam terms
The critical angle c is the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is exactly 90°. At this angle the refracted ray just grazes the boundary surface.
It links to refractive index by n = 1 ÷ sin c, so a denser medium has a smaller critical angle. For example, if n = 1.5, then sin c = 1 ÷ 1.5 = 0.667, giving c ≈ 41.8°.
The angle is stated in degrees (°). A correct sentence: light in the glass strikes the surface at exactly the critical angle, so it refracts along the boundary.
Do not confuse it with total internal reflection
Do not confuse the critical angle with total internal reflection. The critical angle is a specific angle; total internal reflection is the phenomenon that happens only when the angle of incidence exceeds the critical angle.
At the critical angle itself the light refracts at 90°, not reflects fully.
In Paper 2, a calculate question gives the refractive index and expects c = sin⁻¹(1 ÷ n), while a define question wants the wording: the angle of incidence in the denser medium at which the angle of refraction is 90°.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)