From which line are a reflected wave's arrival and departure angles measured?
Fintan aims a laser pointer at a bathroom mirror and wants one number for the beam's angle. Measured from the glass, it reads 60°. Measured from a line sticking straight out of the glass, it reads 30°.
In the waves topic, you met the normal, the line perpendicular to the boundary at the point where the wave strikes. This lesson keeps that line and makes it the reference for every angle in this topic.
The line sticking straight out of the mirror is the normal: the line perpendicular to the surface at the point where the ray strikes.
The angle between the incoming ray and the normal is called the angle of incidence.
The angle between the reflected ray and the normal is called the angle of reflection.
Fintan's beam strikes the mirror at 60° from the surface.
The surface and the normal are 90° apart, so the beam sits 30° from the normal.
So the angle of incidence is 30°, not 60°.
When a problem hands you an angle measured from the surface, subtract it from 90° to get the angle from the normal.
A curved surface has no single direction, but it always has a perpendicular at the strike point, so measuring from the normal works everywhere.
A laser beam strikes a flat mirror at 72° from the mirror's surface. What is its angle of incidence?
The angle of incidence is measured from the normal.
The surface and the normal are 90° apart.
90 − 72 = 18
The angle of incidence is 18°.
A ray arrives at a flat mirror with an angle of incidence of 34°. What angle does the ray make with the mirror's surface?
The 34° is measured from the normal.
The surface and the normal are 90° apart.
90 − 34 = 56
The ray makes 56° with the surface.
A ray of sunlight strikes a store window at 70° from the glass surface. Enter the ray's angle of incidence, in degrees.
A ray hits a flat mirror. One angle lies between the ray and the mirror's surface; another lies between the ray and the normal. Which one is the ray's angle of incidence?
Two rays hit the same flat mirror at one point. Haldis says the ray that runs closer to the normal has the smaller angle of incidence. Is Haldis correct?
Which statement gives the convention for measuring a light ray's angles at a surface?
A flashlight beam strikes a still swimming pool's surface at 55° from the water's surface. Enter the beam's angle of incidence, in degrees.
A ray travels straight along the normal and hits a flat mirror head-on. Enter its angle of incidence, in degrees.