Angle of incidence
Also called Incident angle
The angle of incidence is the angle between an incoming ray and the normal, the line drawn perpendicular to the surface at the point where the ray strikes. It is never measured from the surface itself.
The CED builds the definition into the law of reflection. EK 13.1.B.2: the angle between the incident ray and the normal, the line perpendicular to the surface, is equal to the angle between the reflected ray and the normal.
From the normal. Every time. A ray that grazes a pane of glass at 20 degrees to the glass has an angle of incidence of 70 degrees, because . Reading the wrong one of that pair is the single most common way to lose an optics question, and it does not stay local: the same angle feeds three different results.
| Where it appears | What uses it |
|---|---|
| Law of reflection, 13.1.B.2 | |
| Snell's law, 13.3.A.4 | |
| Critical angle, 13.3.A.5.i | the threshold for total internal reflection |
EK 13.3.A.4 names the pair explicitly, saying Snell's law relates the angles of incidence and refraction to the indices of refraction of the two media, and EK 13.3.A.5.i calls the threshold a critical angle of incidence.
Zero has a name and a consequence. A ray arriving along the normal has an angle of incidence of zero, which is called normal incidence. EK 13.3.A.4.iii: when a light ray is incident along the normal to a surface, the transmitted ray is not refracted. It changes speed on crossing but not direction, so a slab viewed straight on displaces nothing.
The reflection entry has the specular and diffuse cases.