| English | Angle of incidence |
|---|---|
| Bahasa Melayu | Sudut tuju |
| 中文 | 入射角 |
Definition
The angle between an incident ray and the normal at the point where the ray meets a surface.
In exam terms
The angle of incidence i is the angle between the incident ray and the normal at the point where the ray strikes a surface. It is always measured from the normal, not from the surface itself, and is stated in degrees (°).
The normal is the line drawn at 90° to the surface at the point of contact. A correct sentence: the ray hits the mirror at an angle of incidence of 30° to the normal. In reflection the angle of reflection equals the angle of incidence; in refraction the angle of incidence is linked to the angle of refraction by Snell's law.
Do not confuse it with angle of refraction
Do not confuse the angle of incidence with the angle of refraction. The angle of incidence is in the first medium before the ray crosses the boundary; the angle of refraction is in the second medium after crossing.
Both are measured from the normal.
In Paper 2, a diagram question may ask you to mark and label the angle of incidence, draw it between the incoming ray and the normal, never from the surface. Measuring from the surface is a common error that loses the mark.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)