3.4.1 Single Slit Diffraction

Diffraction

• Diffraction is the spreading out of waves when they pass an obstruction
• This obstruction is typically a narrow slit known as an aperture
• The extent of diffraction depends on the width of the gap compared with the wavelength of the waves
• Diffraction is the most prominent when the width of the slit is approximately equal to the wavelength

Diffraction: when waves pass through a narrow gap, they spread out

• Diffraction is usually represented by a wavefront as shown by the vertical lines in the diagram above
• The only property of a wave that changes when it diffracts is its amplitude
• This is because some energy is dissipated when a wave is diffracted through a gap
• The diffraction pattern of light can be represented as a series of light and dark fringes which show the areas of maximum and minimum intensity
• If a laser emitting blue light is directed at a single slit, where the slit width is larger than the wavelength of the light, it will spread out as follows:

The intensity pattern of blue laser light diffracted through a single slit

• If the laser were to be replaced by a non-laser source emitting white light:
• The central maximum would be white
• All maxima would be composed of a spectrum
• The shortest wavelength (violet / blue) would appear nearest to the central maximum
• The longest wavelength (red) would appear furthest from the central maximum
• The fringe spacing would be smaller and the maxima would be wider

Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width

Single Slit Diffraction

• As discussed above, the effects of diffraction are most prominent when the gap size is approximately the same or smaller than the wavelength of the wave
• As the gap size increases, the effect gradually gets less pronounced until, in the case that the gap is much larger than the wavelength, the waves are no longer spread out

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

• If the blue laser were to be replaced with a red laser:
• The wavelength of red light is longer so the light would diffract more
• The intensity fringes would therefore be wider

The intensity pattern of red laser light shows longer wavelengths diffract more than shorter wavelengths

• If the slit was made narrower:
• The intensity would decrease
• The fringe spacing would be wider

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

• 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

Exam Tip

When drawing diffracted waves, take care to keep the wavelength (the distance between each wavefront) constant. It is only the amplitude of the wave that changes when diffracted.

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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