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Diffraction is a cool thing that happens to waves when they hit something in their path. It can be an object or even a little hole. The way the wave moves changes based on how big the object or hole is. This is called diffraction.

The phenomenon of diffraction

When a wave moves towards an object, they interact with each other. Imagine a calm breeze passing over a rock on a lake's surface. The breeze creates waves on the water's surface where there is nothing to stop them. However, right behind the rock, the waves become uneven. The size of the rock affects how uneven the waves become.

If we replace the rock with an open gate, the same thing happens. The wave makes straight lines before the gate. But as it passes through the gate's opening, the wave becomes uneven. This happens because of the edges of the gate.

Single slit aperture

The dimension of the aperture affects its interaction with the wave. In the centre of the aperture, when its length d is greater than the wavelength λ, part of the wave passes through unaltered, creating a maximum beyond it.

A wave passing through an aperture whose aperture length d is greater than the wavelength λ

If we increase the wavelength of the wave, the difference between maximums and minimums is no longer evident. What happens is that the waves interfere with each other destructively according to the width d of the slit and the wavelength λ. We use the following formula to determine where the destructive interference occurs:

n λ = d sin θ

Here, n = 0, 1, 2 is used to indicate the integer multiples of the wavelength. We can read it as n times the wavelength, and this quantity is equal to the length of the aperture multiplied by the sine of the angle of incidence θ, in this case, π/2.  We, therefore, have constructive interference, which produces a maximum (the brighter parts in the image) at those points that are multiples of half the wavelength. We express this with the following equation:

n (λ / 2) = d sin θ

There is a slower transition between a maximum (blue) and a minimum (black) before the aperture
There is a slower transition between a maximum (blue) and a minimum (black) before the aperture

Finally, n in the formula indicates not only that we are dealing with multiples of the wavelength but also the order of the minimum or maximum. When n = 1, the resulting angle of incidence is the angle of the first minimum or maximum, while n = 2 is the second one and so on until we obtain an impossible statement like sin θ must be greater than 1.

Diffraction caused by an obstacle

Our first example of diffraction was a rock in the water, i.e., an object in the way of the wave. This is the inverse of an aperture, but as there are borders that cause diffraction, let’s explore this, too. While in the case of an aperture, the wave can propagate, creating a maximum just after the aperture, an object ‘breaks’ the wave front, causing a minimum immediately after the obstacle.

A wave is generated below the obstacle, with the crests depicted in colour and the troughs in black
Figure 4. A wave is generated below the obstacle, with the crests depicted in colour and the troughs in black

The picture shows a wave moving towards obstacles of different sizes.

The first obstacle is small, so it doesn't stop the wave completely. But it does make the wave uneven. This happens because the obstacle is much smaller than the wave.

The second obstacle is bigger, almost the same size as the wave. This causes the wave to break apart and form a minimum right after the obstacle.

The third obstacle is even bigger, and things get more complicated. The wave front is divided into three parts, and there are two minimums. The next wave front has one minimum, and the crests and troughs are bent.

Basically, when a wave hits an obstacle, it makes the wave uneven. If the obstacle is big enough, it can even change the wave's phase.


What is diffraction?

Diffraction is a physical phenomenon that occurs when a wave finds an aperture or an object in its path.

What is the cause of diffraction?

The cause of diffraction is a wave being affected by an object that is said to be diffracting.

Which obstacle’s parameter affects the diffraction pattern, and what is the related wave’s parameter?

The pattern of diffraction is affected by the width of the object compared to the wavelength of the wave.

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