Video Transcript
Which of the diagrams correctly
shows refraction of a light ray, assuming that the substances the light ray moves
through have different densities?
This is a question about light
entering and then leaving a substance that has a different density to the medium it
was originally traveling through. To answer this question, we need to
identify the diagram that shows the light ray emerging from this gray block in the
correct direction. We’re not shown the path of the
light ray inside the block. So we’ll have to draw it in
ourselves and use our knowledge of refraction to recognize which diagram shows a
valid path for the light ray.
Remember that refraction is the
change of direction of light as it passes from one medium to another. This occurs because light travels
at different speeds in different media. In all four diagrams, the light
passes between different media two times and thus should refract two times. The first refraction occurs when
light enters the gray block. The second refraction occurs when
it exits the gray block. So we know that the light should
change its direction of travel twice. This means we can immediately
eliminate diagram (b).
As we can see, by connecting the
points where the light ray leaves and enters the block, the light doesn’t change
direction either when it enters the new medium of the block or when it leaves the
block. This could only be the case if the
substances the light travels through have the same densities. But we were told that the
substances do have different densities. So, diagram (b) does not correctly
show refraction.
Now, we know that when light is
refracted, it is bent either towards or away from the surface normal. Remember that the normal is simply
the line perpendicular to the surface of the medium boundary. When light enters a less dense
medium and speeds up, it is bent towards the normal. When light enters a more dense
medium and slows down, it is bent away from the normal. Notice that in both cases, light
transmits or passes through the medium boundary and thus crosses the surface normal
at the point where it enters the new medium.
But now take a look at this example
diagram, and notice that here the light is shown as transmitting or passing into the
new medium. But it almost seems to be
reflecting from the surface normal at the point where it enters the new medium. This is not how light refracts. In reality, when light transmits
and refracts, the refracted ray should move through the new medium on the opposite
side of the surface normal as the original, incident ray.
It’s only possible for the
refracted ray to be bent somewhere within this 90-degree range, between the surface
boundary and its normal, but on the opposite side of the normal as the incident
ray. It’s not possible for this ray to
be refracted so that it travels over here, on the same side of the normal as the
incident ray. Keeping this in mind, we can
eliminate diagram (c).
To better understand why (c) is
wrong, let’s draw the path that the light ray takes inside the block. We can see that at both medium
boundaries, the light ray’s path violates these principles we just discussed. This supposed path strangely seems
to be somehow reflected from the surface normal both times. But we know this can’t be right, so
diagram (c) is incorrect.
Note that, in the same way, we can
also deduce that diagram (d) is incorrect. Here, the light ray seems to behave
at the first medium boundary, where it enters the block. But then when the light leaves the
block, it’s not possible for it to actually take this path, since it should transmit
on the opposite side of the surface normal. Thus, diagram (d) is incorrect.
That leaves us with (a), so let’s
take a closer look at this diagram. As the light enters the block, it’s
bent toward the surface normal. This tells us the ray must be
moving from a lower-density medium, the space around the block, to a higher-density
medium, the gray block. When the light ray leaves the
block, it is refracted a second time and bent away from the normal, which is what we
expect, since here the ray is moving back to a lower-density medium.
So diagram (a) represents a
perfectly sensible representation of refraction for these media. Therefore, we know that diagram (a)
shows the correct path of a light ray moving through media of different
densities.
