Video: Defining Diffraction

Which of the following is a correct definition of diffraction? [A] Diffraction is the change in direction of a wave that passes from one medium into another with a different density. [B] Diffraction is the change in wavelength of a wave that passes through an aperture. [C] Diffraction is the change in direction of a wave that passes close to an object and changes its direction by an angle greater than 90 degrees. [D] Diffraction is the change in speed of a wave that passes through an aperture. [E] Diffraction is the change in direction of a wave that passes close to an object and changes its direction by an angle smaller than 90 degrees.

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Video Transcript

Which of the following is a correct definition of diffraction?

Now, this question gives us five possible definitions. So let’s go through them one by one and see which one is correct.

Number one: Diffraction is the change in direction of a wave that passes from one medium into another with a different density.

Okay, so let’s draw a diagram of what that looks like. Let’s say that this is a boundary between two media. Let’s also say that our wave is initially travelling in the blue medium and then moving in this direction. It arrives at an angle to the normal, to the surface. And then when it meets the boundary, it changes direction. Whereas, if it hadn’t changed direction, it would’ve carried on travelling in this direction. Now, the two media in question have different densities. So let’s say that this bottom medium has a density 𝜌 one. And this one has a density 𝜌 two. Now, this scenario does not look like diffraction. In fact, it looks more like refraction. Refraction is the bending of a wave when it passes from one medium to another, where the second medium has a different density to the first. Therefore, this first statement is not the correct definition of diffraction. So we can move on to definition number two.

Number two says that diffraction is the change in wavelength of a wave that passes through an aperture.

Okay, let’s draw what that would look like. Now, here’s a diagram of our aperture. This here could be a slit or a hole or something like that. In other words, an aperture. And let’s say we’ve got a wave travelling in this direction. So here comes the wave, and it meets the aperture. Now this statement says that the wavelength of the wave changes as it passes through the aperture. Well, let’s say that this wave that’s coming in is a light wave. We know that for light, the wavelength is directly linked to the colour of the light. That’s how our eyes perceive wavelength. So if the wavelength of the wave is changing as it passes through the aperture, then so must its colour. Now, is this something that we see when we have, for example, a laser beam passing through an aperture? Well, no. We don’t see a wavelength change or, in fact, a colour change when we conduct this experiment in real life. So definition two is not the right answer either. Let’s move on to number three then.

This statement says that diffraction is the change in direction of a wave that passes close to an object and changes its direction by an angle greater than 90 degrees.

Once again, let’s draw a diagram. Let’s say that this is our object, whatever it may be. And here’s our incoming wave. Well, this wave is simply meant to pass close to the object. Hence, we’ve drawn it like this. Now, initially, it’s moving in this direction. And this statement is saying that the wave changes its direction by greater than 90 degrees. So here’s a 90-degree angle. And this definition is saying that the outgoing wave will be going in something like this direction. This angle is greater than 90 degrees. And it looks like some sort of weird reflection kind of thing rather than diffraction. And this is a really weird thing because it’s only passing close to an object rather than coming in contact with an object. So this is more reflection than it is diffraction. And therefore, this is not the right definition either. Let’s move on to the next one.

Number four: Diffraction is the change in speed of a wave that passes through an aperture.

We know the drill by now. Let’s draw a diagram. Here is our aperture. And here’s our incoming wave. Now this statement says that the speed of the wave changes when this wave passes through an aperture. But let’s recall something. We can recall that the speed of a wave, 𝑣, is given by multiplying the frequency of that wave, 𝑓, by the wavelength of that wave, 𝜆. Now, if we’ve just got a wave travelling and we’re not considering something like the doppler effect, then the frequency has to stay the same because this frequency is determined by what the source of that wave is. So we’ve just said that the frequency must stay the same. But the speed of the wave is changing as it passes through the aperture. And so if the speed is changing, then so must the wavelength. So a wave that’s changing speed must change wavelength as well. And therefore, we’d once again see a colour change. Now as we said earlier, this is not something that we see when we’re studying diffraction. So yet again, this is not the right definition. Now there’s only one more left. So that must be the right one. So let’s go through it and make sure it is.

So number five says that diffraction is the change in direction of a wave that passes close to an object and changes its direction by an angle smaller than 90 degrees.

Once again, here’s our object. And here is our incoming wave. This definition says that the wave changes direction as it passes close to an object. So it does something like this. And of course, this angle here between the incident and the outgoing wave is less than 90 degrees. Now this really is something that we see when diffraction occurs. When a light wave passes close to an object, we can see a change in direction. And it’s this effect that’s responsible for many others, such as, for example, when a wave passes through an aperture. What we see is a supposed bending of light that gives us this really cool-looking diffraction pattern. So this final definition is a correct definition of diffraction.

And so, our final answer is that diffraction is the change in direction of a wave that passes close to an object and changes its direction by an angle smaller than 90 degrees.

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