A student investigates the refraction of light as it passes from a semi-circular acrylic block into air.
They record the angle of incidence, iii, and the angle of refraction, rrr, for several trials. Their data is plotted on the graph in Figure 1.

Describe how the student should use the graph in Figure 1 to determine the critical angle for this acrylic block.
Practise Edexcel GCSE Physics Reflection, refraction and lenses with exam-style questions for Foundation and Higher tier. 100 questions, matched to the Edexcel GCSE Physics (1PH0) specification and written in Paper 1 and Paper 2 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.