Diffraction

This lesson covers:

  1. How waves can be represented as wavefronts
  2. What diffraction is
  3. How wavelength and gap size affect diffraction through a gap
  4. How wavelength affects diffraction at an edge

Waves can be represented as wavefronts


In order to understand diffraction, you need to know that waves can be represented as wavefronts. A wavefront is effectively a line connecting the same point on each wave. In the diagram below we have drawn 3 wavefronts corresponding to the 3 peaks of the transverse waves. 


The distance between two adjacent wavefronts is the wavelength. 

Diagram showing wavefronts and direction of wave travel.

What is Diffraction?


Diffraction is the phenomenon where waves spread out and change direction as they pass through a gap or around an obstacle.


Examples of diffraction include:

  • Sound waves spreading out as they pass through a doorway.
  • Water waves spreading out as they pass through the gap in a harbour wall.
Diagram showing wavefronts bending and spreading out as they pass through a gap with a barrier.

Factors that affect the amount of diffraction


The amount of diffraction depends on the relationship between two factors:

  1. The wavelength of the wave
  2. The width of the gap
Diagram showing diffraction with gap width greater than wavelength and gap width equal to wavelength.
  • The smaller the difference between the gap width and wavelength, the greater the amount of diffraction. 
  • When there is a significant difference between the gap width and the wavelength, very little diffraction occurs.
  • The most diffraction occurs when the wavelength is equal to the gap width. 

Diffraction at edges


Waves also undergo diffraction as they pass the edge of an object. 


To see how wavelength affects the amount of diffraction, we can compare radio waves (very long wavelength) to microwaves (shorter wavelength).

Illustration showing radio waves undergoing diffraction as they pass the edge of a hill and reach a house.

As radio waves have a long wavelength, they diffract a lot as they pass over the hill. 

This allows the radio waves to reach the house.

Illustration showing microwaves undergoing diffraction as they pass the edge of a hill.

Microwaves have a much shorter wavelength though.

This means that they experience less diffraction, and so don't reach the house. 


Which of the following affects the amount of diffraction through a gap?

the amplitude of the waves

the wavelength of the waves

the size of the gap

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What happens to waves as they pass the edge of an object?

they absorb into the edge

they diffract around the edge

they refract through the edge

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Which of the following would result in the most diffraction?

a gap size small than the wavelength

a gap size equal to the wavelength

a gap size much bigger than the wavelength

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What is meant by the term 'diffraction'?

The spreading out and bending of waves as they travel through a vacuum

Waves which have oscillations perpendicular to the direction of wave motion

The spreading out and bending of waves as they pass a gap or an edge

Waves which can travel at the speed of light

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Which of the following would result in the most diffraction?

Diagram showing three different slit widths labeled A, B, and C to demonstrate diffraction.

A

B

C

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