In diagram 1, total internal reflection occurs at Q - Edexcel - GCSE Physics - Question 3 - 2013 - Paper 1

Total internal reflection occurs at point Q because the angle of incidence of light in glass exceeds the critical angle, which is approximately 42 degrees. When light travels from a denser medium (glass) to a less dense medium (air), it must strike the boundary at an angle greater than the critical angle for total internal reflection to take place. Additionally, glass has a higher refractive index than air, which reinforces this condition for total internal reflection.