Total Internal Reflection
2 marks · Easy| Angle of incidence (i) | Angle of refraction (r) |
|---|---|
| 30° | 19° |
| 40° | 27° |
| 42° | 90° |
Using the table above, identify the critical angle for this glass–air boundary and explain how you know.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
The critical angle is 42°, because when the angle of incidence is 42°, the angle of refraction is 90°, showing that this is the minimum angle for total internal reflection.
Also accept: 42°; The angle at which the angle of refraction is 90°
Common mistakes: Picking wrong angle based on incidence instead of refraction.; Not mentioning why.
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Identifies the critical angle as 42° from data
B1 · 1 mark · Explains this is because the angle of refraction is 90° at this incidence angle.
Worked solution
The critical angle is 42° as shown in the table. At this angle of incidence, the refracted ray travels along the boundary (angle of refraction is 90°), which defines the critical angle.