A 45^o prism can turn a ray through 90^o or through 180o

When drawing these diagrams use a ruler and a protractor and ensure your digrams are neat and large enough to label accurately. Always tell the examiner what is happening at each boundary.

Turning a ray through 90^o

• At boundary A the angle of incidence is 0^o therefore the ray goes through undeviated.
• At boundary B the angle of incidence is 45^o therefore the ray undergoes total internal reflection as it is a bigger angle than the critical angle of glas (42^o).
• At boundary C the angle of incidence is 0^o therefore the ray goes through undeviated.

Turning a ray through 180^o

• At boundary A the angle of incidence is 0^o therefore the ray goes through undeviated.
• At boundary B the angle of incidence is 45^o therefore the ray undergoes total internal reflection as it is a bigger angle than the critical angle of glas (42^o).
• At boundary Cthe angle of incidence is 45^o therefore the ray undergoes total internal reflection as it is a bigger angle than the critical angle of glas (42^o).
• At boundary D the angle of incidence is 0^o therefore the ray goes through undeviated.

Don't forget that each segment of the ray needs an arrow (only one!) and don't forget to construct normals where the ray hits the boundary.

The advantage of using a prism rather than mirrors to change the path of the ray is that 100% of the light energy changes direction - there is no absorption at each boundary as there would be with normal reflection.

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