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GCSE questions on refraction and TIR

Q5. Some students used water waves in a ripple tank to model the behaviour of light waves.

The diagram shows what happens to the wave fronts as they pass the boundary between deep water and shallower water.

(a) Explain why refraction happens at the boundary between the deep water and shallower water.

The velocity/speed of the wavefronts is slower in shallow water so edge of wavefront entering shallow water slows down but the part of the wavefront that is still in deeper water continues at a higher speed. This leads to a change in direction of the wavefronts as one part of the wavefront changes speed before other part.

[3 marks]

(b) The next diagram shows the wavefronts travelling parallel to the boundary between deep water and shallower water. Explain why the wavefronts do not refract at the boundary.

As the wavefront is parallel to the boundary every point on the wavefront enters the shallow water at the same time and so every point slows down at the same time.

[2 marks]

(Total 5 marks)