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Edexcel A Level Physics:復(fù)習(xí)筆記5.25 Diffraction

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年10月10日下午4:00

Diffraction

Diffraction is the?spreading out?of waves when they pass an obstruction

Diffraction through a gap

This obstruction is typically a narrow gap (a slit, or aperture)

$Diffraction-Wavefronts$

Diffraction is usually represented by a wavefront as shown by the vertical lines in the diagrams above
The only property of a wave that changes when its diffracted is its?amplitude
- This is because some energy is?dissipated?when a wave is diffracted through a gap

Diffraction around an obstacle

The diffraction pattern for a large slit can be thought of as a wave passing two completely separate obstacles
- This shows that when a wave meets an obstacle a diffraction pattern forms around the edges.
- Behind the obstacle a ‘shadow’ forms where no part of the wave reaches

Factors that affect diffraction

The effects of diffraction are most prominent when the gap size or obstacle is approximately the same or smaller than the wavelength of the wave
- As the size of the gap or obstacle increases, the effect gradually gets less pronounced
- When the gap is much larger than the wavelength, the waves are no longer spread out

$8.2.1.2-Diffraction-gap-size$

The size of the gap (compared to the wavelength) affects how much the waves spread out

Explaining diffraction

Huygens developed a model for wave propagation which suggested that every point on a?wavefront?can be considered to be a point source of secondary waves (which he called wavelets)
- This leads to a diagram, called?Huygens’ construction, which shows that new wavefronts are tangential to the secondary wavelets
- The tangents create the curve of the new wavefront emerging either through the gap or around the obstacle

$5-25-waves-diffraction-around-an-obstacle_edexcel-al-physics-rn$

When a wave meets an obstacle a diffraction pattern forms around the edges, with a ‘shadow’ created behind the obstacle where no part of the wave reaches

Huygens' Construction for Diffraction Through a Gap?

5-25-waves-huygens-construction_edexcel-al-physics-rn

Those point sources which pass through the gap create new wavelets on the other side, leading to the characteristic curved shape of the diffracted wave

Huygens' Construction for Diffraction Around an Obstacle?

5-25-waves-huygens-construction-around-obstacle_edexcel-al-physics-rn

Those point sources which pass around the obstacle create new wavelets on the other side, leaving empty space where the 'shadow' is seen

Worked Example

When a wave is travelling through air, which scenario best demonstrates diffraction?

A.? ?UV radiation through a gate post

B.? ?Sound waves passing a steel rod

C.? ?Radio waves passing between human hair

D.? ?X-rays passing through atoms in a crystalline solid

Answer:? ? D

Diffraction is most prominent when the wavelength is close to the aperture size
UV waves have a wavelength between 4 × 10-7?– 1 × 10-8?m so won’t be diffracted by a gate post
Sound waves have a wavelength of 1.72 × 10-2?– 17 m so would not be diffracted by the diffraction grating
Radio waves have a wavelength of 0.1 – 106?m so would not be diffracted by human hair
X-rays have a wavelength of 1 × 10-8?– 4 × 10-13?m which is roughly the gap between atoms in a crystalline solid
- Therefore, the correct answer is?D