Video Transcript
Which of the following sets of
conditions best describes total internal reflection? (A) Total internal reflection
occurs when a light ray is completely reflected at the boundary between two media of
different densities and the light ray is traveling in the higher-density medium. (B) Total internal reflection
occurs when a light ray is completely reflected at the boundary between two media of
different densities and the light ray is traveling in the lower-density medium. (C) Total internal reflection
occurs when a light ray is completely absorbed in an object. (D) Total internal reflection
occurs when the angle of reflection of the reflected light ray is equal to the angle
of refraction of the refracted ray. Or (E) total internal reflection
occurs when the reflected light ray moves along the same path as the incident light
ray but in the opposite direction.
Okay, we need to recall how total
internal reflection, or TIR, is produced in a light ray. To determine which of these answer
options is correct, let’s go through them all, starting with (E) and working our way
up.
Option (E) suggests that total
internal reflection occurs when a reflected light ray travels along the same path as
the incident ray. In order for this to happen, a
light ray would have to be reflected at a surface that the ray is incident upon
normally or perpendicularly. If this is the case, then the angle
of incidence is zero degrees, since the angle of incidence is measured with respect
to the surface normal. We should recall that total
internal reflection occurs only if the angle of incidence is greater than the
critical angle. And if the angle of incidence is
zero degrees, then it can’t be greater than the critical angle. Let’s eliminate this answer
choice.
Moving on, option (D) suggests that
total internal reflection occurs when the angle of reflection is equal to the angle
of refraction. But this doesn’t make sense. If a ray is experiencing total
internal reflection, then it does not transmit and refract. The ray only gets reflected, and
thus there is no refracted ray. Option (D) cannot be correct
then.
Next, (C) is talking about a light
ray being completely absorbed in an object. If a light ray gets totally
absorbed, then it cannot reflect. So this cannot describe total
internal reflection.
Finally, options (A) and (B) are
similar to each other. They both suggest that total
internal reflection occurs when a light ray is completely reflected at the boundary
between two media of different densities. And this much is true. But to decide which one is the best
description of total internal reflection, we need to remember that it occurs with
the light ray traveling in the higher-density medium. Total internal reflection cannot
occur for a light ray that’s traveling in a lower-density medium and incident upon
the surface of a higher-density medium. Thus, we eliminate answer choice
(B), and we know that (A) is correct.
Total internal reflection occurs
when a light ray is completely reflected at the boundary between two media of
different densities and the light ray is traveling in the higher-density medium.
