### Video Transcript

The diagram shows the path of a
light ray through a triangular prism. What is the sum of the angles 𝑎
and 𝑏?

In this question, we need to work
out the sum of these two angles, 𝑎 and 𝑏. We can see that both 𝑎 and 𝑏 are
angles between one surface of the prism and one of these dashed lines. Recall that these dashed lines are
called normals. When we draw ray diagrams like
this, we often draw normal lines to help us visualize the path of the light
correctly. Crucially, normals are always drawn
normal to the surface of the prism that the light is passing through.

Remember that normal just means
perpendicular to. This means that the angle between
each normal and the corresponding surface of the prism is simply 90 degrees. Hence, both 𝑎 and 𝑏 are equal to
90 degrees. So, the sum of 𝑎 and 𝑏 is just 90
degrees plus 90 degrees. This works out as 180 degrees. The sum of the angles 𝑎 and 𝑏 is
180 degrees.