Total Internal Reflection
3 marks · NormalDescribe an experiment using a semi-circular glass block and a ray box to determine the critical angle of glass. State the steps you would take and the measurements you would make.
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Common mistakes: Missing clear identification of when total internal reflection starts; Forgetting to measure angle at boundary
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Directs ray through curved side to hit flat side at boundary between glass and air.
M2 · 1 mark · Varies angle of incidence and observes the appearance and disappearance of the refracted and totally internally reflected rays.
M3 · 1 mark · Measures the smallest angle at which the light inside is totally internally reflected or refracted along the boundary (the critical angle).
Worked solution
Shine a ray of light from the ray box into the center of the curved face of the semi-circular block so it strikes the flat edge at the glass–air boundary. Gradually increase the angle at which the ray strikes the flat edge. Observe the point where the ray, instead of refracting out, just disappears from the air side and is totally reflected along the inside boundary (this is when the angle equals the critical angle). Measure this angle of incidence from the normal at the flat surface—the result is the critical angle for glass.