Video Transcript
Which of the following is the term
used to refer to the separation of white light into its component wavelengths due to
the variation of the refractive index with the wavelength of an object that the
white light passes through? (A) Aberration, (B) diffraction,
(C) dispersion, (D) distortion, (E) deviation.
Okay, so in this exercise, we’re
looking for a word that defines or describes this process here. We’re told that what’s taking place
is white light is incident on some object. And then, because the index of
refraction of that object varies with wavelength, it refracts the different colors
differently, so they spread out like this. And we want to identify a term from
among these five that describes this process.
We can start from the top and work
our way down, first aberration. This term describes something about
an optical system that causes it to form images in an imperfect way. It could be, for example, that a
lens, say, has an aberration. And this makes it refract light
improperly, leading to a low quality image or even no image at all. So this term aberration does have
to do with light, but not in the way we’re describing it here. So we’ll cross this option off our
list.
Next, we get to diffraction. This is a property of light that
describes how it spreads out when passing through a narrow opening or around a
barrier. For example, if we send coherent
light waves through a small opening like this, then diffraction makes those waves
spread out through the opening like this. So this term does describe the
spreading out of waves of light, but it’s not because of a variation of refractive
index in a material. So this term isn’t quite the right
one either.
Next, we get to dispersion, and
this term agrees completely with what’s written in our question statement. When an object like this prism over
here separates out white light according to its component wavelengths, in this case
colors, because the refractive index of that prism varies with wavelength, then that
is known as dispersion. So option (C) looks like it’ll be
our answer. But just to make sure, let’s check
options (D) and (E). Option (D) suggests distortion as
the word for describing this process. But, in a physics context, this
word typically means something different.
Say that we have a wave of light
and we want to amplify the wave. If we do that and the amplified
wave comes out looking like this, we say there’s been amplitude distortion or simply
distortion of this wave. So this term distortion is not a
match for describing white light being separated into its component wavelengths. And lastly, let’s think of this
term deviation. Now, deviation actually is related
to the way a prism interacts with light. It describes how light is refracted
at this face here and then at this face here of the prism in such a way that its
final trajectory, say in the case of our beam of red light, is deviated by some
amount from the ray’s original direction.
However, this term does not
describe how the refractive index of an object can vary with wavelength. And, therefore, though it describes
how a particular wavelength of light is deviated as it passes through some object,
it doesn’t explain the separation of white light into its component wavelengths. So we won’t choose option (E)
either. And this confirms to us that option
(C), dispersion, is the correct choice. This term refers to the separation
of white light into its component wavelengths due to the variation of the refractive
index with the wavelength of an object the white light passes through.
