45^{o}
Prisms used to change a ray's direction
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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