Video Transcript
Which of the following statements most correctly describes the effect of white light
passing through a gap about 550 nanometers wide? (A) The white light is not diffracted. (B) The different colors in the white light are diffracted equally. (C) The different colors in the white light are diffracted by different amounts. Or (D) some of the colors in the white light interfere destructively and are not seen
in the diffraction pattern.
In this question, we’re asked to choose the statement that most correctly describes
the effect of white light passing through a gap with a width of 550 nanometers. Let’s begin by reminding ourselves about what white light is and how it diffracts as
it passes through a narrow gap.
White light refers to light waves that are made up of all the different wavelengths
of visible light. We can see this by shining white light onto a prism. When white light is incident on a prism, it undergoes dispersion and separates into
different colors. Every color that makes up white light has a different wavelength, with red light
having the longest wavelength and violet having the shortest wavelength.
Now that we know what white light is and that different colors of light have
different wavelengths, let’s look at the diffraction of light through a gap. When light waves are incident on a barrier that has a narrow gap in it, the light
waves that pass through the gap will be diffracted on the opposite side of the
barrier.
Diffraction refers to light changing direction without being incident on a boundary
of varying refractive indices. The angle at which light waves will be diffracted depends on a few things, including
the width of the gap as well as the wavelength of light. If the wavelength of a light passing through the gap is similar to the width of the
gap, the light waves are diffracted by a large amount. They become almost semicircular on the other side of the gap. But if the wavelength of light is much smaller than the width of the gap, the wave
barely diffracts at all, with only noticeable diffraction occurring near the ends of
the gap.
Since white light is made up of lots of different wavelengths of light, we can
imagine that the longer wavelengths will be diffracted more, while the shorter
wavelengths will be diffracted less. Importantly, the question tells us the gap is 550 nanometers wide. The range of wavelengths in visible light is around 400 nanometers for violet light
to around 700 nanometers for red light. These wavelengths are quite similar to the width of a gap. So they will all be diffracted, though each wavelength will be diffracted by a
different amount as we’ve already seen.
With this knowledge, let’s take a look at the answer options we’ve been given.
Option (A) states that white light is not diffracted when it passes through the
gap. But we know that all light waves that pass through a narrow gap are diffracted.
Option (B) states that all the colors in the white light are diffracted equally. But we know that the wavelength affects how much light is diffracted. And white light is made up of many wavelengths. So this cannot be correct.
Option (C) states that the different colors of white light are diffracted by
different amounts. We know that wavelength affects the diffraction of light. So this correctly states how white light behaves when diffracted through a narrow
gap. Therefore, the third option is the correct answer. But let’s also take a look at option (D).
Option (D) states that some of the colors in the white light interfere destructively
and are not seen in the diffraction pattern. However, we can recall that in order for waves to interfere destructively, they must
be consistently out of phase with each other. This only occurs when the wavelengths of the two waves are the same. In other words, different wavelengths of light, or different colors, cannot
consistently interfere destructively with each other. And therefore option (D) is also incorrect.
Our final answer is option (C). The different colors in the white light are diffracted by different amounts.
