Video Transcript
When a light ray passes from a
medium into a different medium of lower density, does the speed of the ray increase,
decrease, or not change?
This question is about how the
density of a medium, or a substance, affects the speed of a light ray in that
medium. Thus, we need to recall that light
travels at different speeds in different media. Also, recall that media is the
plural of medium.
Now, many people are familiar with
the term “the speed of light.” But this typically only refers to
the speed of light in a vacuum, such as empty space. For instance, light actually
travels a tiny bit slower in air. Light travels slower than this,
still, in water and significantly slower in diamond.
Knowing this, we might recognize a
trend here. In these different media we just
mentioned, light travels slower in more dense substances. It’s typically true that the
greater the density of a medium, the slower the speed of light in that medium. Equally true then is that the lower
the density of a medium, the faster the speed of light. This might make sense intuitively
if we consider the generalization that something that’s more dense is more difficult
to travel through. For example, it’s a lot easier to
walk through air than it is to walk through water. And we know that air is much less
dense than water.
A similar principle applies to a
light ray because the density of a material is often directly related to the
material’s refractive index. Recall that a refractive index,
represented by a lowercase 𝑛, is a measure of how much slower light travels through
a certain medium compared to in a vacuum. On screen, let’s show the
refractive indices of the four different media we mentioned at the beginning of the
video.
Higher-density materials usually
have large refractive indices. The greater the refractive index,
the slower light travels through the material. Likewise, lower-density materials
usually have smaller refractive indices. And the lower the refractive index,
the faster light travels through the material.
Imagine, for instance, that we have
a ray of light approaching a diamond. The light initially moves through
air. Once light passes into the diamond,
which we know is much more dense than air, the light will decrease in speed. Because of the shape of the
diamond, the ray will change direction and reflect inside of it before emerging. And then when the light passes back
into air, its speed will increase because it’s traveling into a much less optically
dense material.
So, in this question, we’ve been
asked “When a light ray passes from a medium into a different medium of lower
density, does the speed of the ray increase, decrease, or not change?” And we know that the correct answer
is “increase.”
