Video Transcript
The diagram shows the path of a light ray through a triangular prism. Which of the following is the physical significance of the angle 𝛼? (A) The angle 𝛼 is directly proportional to the ratio of the base of the prism to
the height of the prism. (B) The angle 𝛼 is directly proportional to the refractive index of a prism. (C) The angle 𝛼 is the critical angle of the prism for total internal
reflection. (D) The angle 𝛼 is the angle through which the direction of the path of a light ray
changes due to the ray passing through a prism. (E) The angle 𝛼 is the angle that a prism would have to rotate around its apex in
order to produce total internal reflection of the incident ray shown.
In this question, we are being asked to look at the angle 𝛼 labeled in the
diagram. To understand what this represents, let’s first recall how the path taken by light
changes as it enters a medium with a different refractive index. When the light passes from the surrounding air into the prism, the refractive index
increases. Hence, the ray of light will refract toward the line that is normal to the surface of
the prism.
However, as the light leaves the prism, the refractive index decreases again. This means that the ray of light refracts away from normal at the point where it
comes out. We can see that when a ray of light passes through the prism, its total deviation is
due to two refraction events: one when it enters and one when it leaves.
We can find the total angle of deviation by adding dotted lines representing the
direction in which the ray enters the prism and the direction in which it leaves the
prism. This angle happens to be 𝛼, the angle we’ve been asked about in the question.
Therefore, from our answer options, we can see that the angle 𝛼 is the angle through
which the direction of the path of a light ray changes due to the ray passing
through a prism. We choose option (D) as our final answer.
