12.2 - Diffraction

Diffraction is waves spreading at gaps and edges

• Diffraction is the spreading out of waves as they pass through a gap or round the edge of an obstacle.
• All waves diffract, though the effect isn't always easy to see.
• As a wave spreads out, its energy is shared over a larger area, so its amplitude decreases.

Diffraction around objects

• Waves diffract around the edges of objects, leaving a shadow behind the object where the wave is blocked.
• The wider the object is compared with the wavelength, the less the wave diffracts around it.

Observing diffraction in a ripple tank

1. Set up the ripple tank so a dipper produces straight wavefronts travelling across the water.
2. Place a barrier in the water. As the waves pass the end of the barrier, they bend round its edge into the region behind it.
3. Place two barriers with a small gap between them. As the waves pass through the gap, they spread out on the far side.

Wavefront diagrams

• A wavefront is an imaginary line joining points on a wave that are at the same stage of their cycle, such as along a crest.
• Wavefronts are drawn at right angles to the direction the wave is travelling.
• The distance between one wavefront and the next is one wavelength.
• Straight wavefronts arriving at a gap or obstacle curve as they spread out.

Diffraction depends on wavelength and gap size

How much a wave diffracts depends on its wavelength and on the size of the gap it passes through.

Diffraction around an edge

The longer the wavelength, the more a wave diffracts round the edge of an object.

Diffraction through a gap

For a gap, diffraction depends on the size of the gap relative to the wavelength:

• Gap much wider than the wavelength - little diffraction; the wave carries on almost straight.
• Gap a bit wider than the wavelength - the wave diffracts only at the edges.
• Gap about the same size as the wavelength - the wave diffracts the most and spreads out fully.

Why light is hard to diffract

Light has a very small wavelength (about 0.0005 mm), so it only diffracts noticeably when the gap is extremely small.