May 2021

Diffraction of Waves: Single Slit & Multiple Slit Diffraction

May 14, 2021

Physics Tuition, Physics Tuition SingaporeWe all know that waves are something we encounter in our everyday lives. From light waves to sound waves, they help us see and hear things on a daily basis. But, there are many things you may not know about how these waves behave – and one of those phenomena is diffraction.

Forming a shadow of an object or the bending of light at the corners of the door are real-life examples of diffraction. Read on to learn more about the diffraction of waves!

 What is diffraction?

When waves come into contact with an obstacle or go through a slit, it exhibits a behaviour called diffraction. Diffraction is defined as the bending of waves around the corners or opening of an obstacle, and it can happen to any forms of waves, including water waves, light waves, and sound waves.

Diffraction can be demonstrated by placing an obstacle in a ripple tank and observing the water wave’s path. You will notice that the waves that pass the object are disturbed. The amount of diffraction that occurs increases when the wavelength of the wave increases.

For example, water wave diffraction is observed when waves bend around small boats, and the water behind them is disturbed. However, the same waves are unable to diffract around bigger boats since their wavelength is smaller than the boat.

Now that you have a basic understanding of what diffraction is, let’s find out a little more about single slit and multiple slit diffraction.

 The single slit experiment

The bending phenomenon of diffraction can be observed in a single-slit diffraction experiment where the wave from a source interferes with itself and produces a distinctive pattern called the diffraction pattern. In single slit diffraction, the diffraction pattern is determined by the wavelength and by the length of the slit. Decreasing the size of the opening can increase the amount of diffraction. To get true diffraction, the slit has to be the size of lambda, the size of the wavelength.

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 Multiple slit diffraction

When waves pass through a large number of evenly spaced parallel slits, diffraction grating occurs where an interference pattern is created. For light, diffraction grating works both for light transmission and reflection. The pattern of interference consists of bright and dark fringes that are sequential. The dark fringes are the product of destructive interference, and the bright fringes are the result of constructive interference.

Some everyday examples where we see multiple slit diffraction of light are surfaces like the back of a CD, or minerals like pearl, opal, or the inner surface of oyster shells. These appear to have different combinations of colours when viewed from different angles, a direct manifestation of multiple slit diffraction.

 Comparing single slit and multiple slit diffraction

The diffraction pattern in the single slit experiment is very much like the multiple slit interference, except instead of multiple slits, the wavefront itself splits into a bunch of adjacent point sources that interact with each other.

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 What happens when two waves meet?

Wave interference is a phenomenon that happens when two waves meet while travelling along with the same medium. An interesting pattern is formed in a ripple tank when there is an interference of two sources of periodic and concentric waves of the same frequency. The light waves from two sources interfere whenever the thick line from one light source meets another thick line from the other sources. The same thing happens with the thin lines. The diagram below shows the pattern of interference caused by two periodic disturbances.

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Diffraction is a phenomenon that occurs everywhere around us. Now that you have a better understanding of it, try noticing some during your day! If you found this interesting, sign up for A level physics tuition classes to learn more! We also provide O level physics tuition for those who have their exams coming.

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