Total Internal Reflection
1 mark · EasyState what is meant by the term ‘critical angle’ when referring to light travelling from glass into air.
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The critical angle is the smallest angle of incidence in glass at which light is totally internally reflected at the glass–air boundary.
Also accept: It is the angle of incidence in glass at which the angle of refraction in air is 90°.
Common mistakes: Confusing critical angle with angle of refraction.; Thinking it is the maximum angle for total internal reflection.
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Marking points
B1 · 1 mark · Gives correct definition of critical angle as minimum angle for total internal reflection.
Worked solution
The critical angle is the smallest angle of incidence (in the denser material) at which total internal reflection occurs. At this angle, the angle of refraction is 90° and the light travels along the boundary.