# Diffraction of Waves

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11.2Diffraction of Waves

In these revision notes for Diffraction of Waves, we cover the following key points:

• What is diffraction?
• What are the conditions for diffraction to occur?
• What happens to the shape of waves in diffraction?
• What is the relationship between diffraction and interference?
• What does Huygens Principle say on diffraction of waves?

## Diffraction of Waves Revision Notes

By definition, diffraction is the process by which a wave is spread out as a result of passing through a narrow aperture or across an edge, typically accompanied by interference between the waveforms produced.

The condition to obtain diffraction is that the dimensions of aperture or of the obstacle must be comparable to wavelength. When the aperture is much larger than the wavelength, no diffraction occurs and when the aperture is smaller than wavelength, circular wavefronts are produced. If the aperture enlarges, waves straighten because they experience diffraction only at the edges of aperture. The same phenomenon occurs when a wave encounters a small obstacle as well.

Diffraction of sound waves enables us to hear even when the speaker is round a corner of a building. This is because sound waves produced by the speaker bend around small obstacles such as the building walls.

Diffraction and interference are related concepts as interference is produced when diffraction from two or more openings does occur. Diffraction from two or more sources produce interference but interference cannot produce diffraction. Therefore, the relationship between diffraction and interference is unilateral.

The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength for a constant opening. In fact, when the wavelength of the mechanical wave is smaller than the obstacle, no noticeable diffraction occurs.

The Dutch scientist Christiaan Huygens developed a useful technique for determining in detail how and where waves propagate during diffraction. Starting from some known position, Huygens's principle states that every point on a wave front is a source of wavelets that spread out in the forward direction at the same speed as the wave itself. The new wave front is tangent to all of the wavelets.

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