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 C 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 D the angle of incidence is 0^o therefore the ray goes through undeviated.

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.