Diffraction
- Diffraction is:
the spreading out of waves after they pass through a narrow gap or around an obstruction
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- When waves meet a narrow gap they must curve to pass through
- The extent of their diffraction depends on the width of the gap compared to the wavelength of the waves
Diffraction: after passing through a narrow gap, the waves curve as they spread out
- The wavefronts of the wave represent the crests and troughs
- The diagram shows what happens to the wavefronts as they pass through the narrow gap
Wavefronts and rays for transverse waves travelling in a horizontal plane
- The only property of a wave that changes when it diffracts is its amplitude
- The wavelength of the wave remains the same
- The amplitude of the diffracted waves is less than that of the incident waves since energy is distributed over a larger area
- Imagine trying to squeeze yourself through a narrow gap. How do you do it?
- You make yourself narrower
- A wavefront will do the same
- The greater the wavelength of the wave, the greater its diffraction
- Examples of diffraction include:
- Radio waves moving in between or around buildings
- Water waves moving through a gap into a harbour
Single Slit Monochromatic Diffraction Pattern
- The diffraction pattern of light passing through a single slit, like a double slit, is a series of light and dark fringes on a faraway screen
- The bright fringes are also areas of maximum intensity, produced by the constructive interference of each part of the wavefront as it passes through the slit
- The dark fringes are also areas of zero or minimum intensity, produced by the destructive interference of each part of the wavefront as it passes through the slit
- The central maximum is:
- Much wider and brighter than the other bright fringes
- Much wider than that of the double-slit diffraction pattern
- On either side of the wide central maxima are much narrower and less bright maxima
- These get dimmer as the order increases
Single Slit Monochromatic Intensity Pattern
- If a laser emitting blue light is directed at a single slit, where the slit width is larger than the wavelength of the light, its intensity pattern will be as follows:
The intensity pattern of blue laser light diffracted through a single slit
- The features of the single slit intensity pattern are:
- The central bright fringe has the greatest intensity of any fringe and is called the central maximum
- The dark fringes are regions with zero intensity
- Moving away from the central maxima either side, the intensity of each bright fringe gets less
Single Slit Diffraction and Intensity Patterns of White Light
- A source of white light diffracted through a single slit will produce the following diffraction pattern:
- It is different to that produced by a double slit or a diffraction grating
- The central maximum is bright white because constructive interference from all the colours happens here:
- Much wider and brighter than the other bright fringes
- Much wider than that of the double-slit diffraction pattern
- All other maxima are composed of a spectrum
- Separate diffraction patterns can be observed for each wavelength of light
- The shortest wavelength (violet / blue) would appear nearest to the central maximum
- The longest wavelength (red) would appear furthest from the central maximum
- The colours look blurry and further away from the central maximum, the fringe spacing gets so small that the spectra eventually merge without any space between them
- As the maxima move further away from the central maximum, the wavelengths of blue observed decrease and the wavelengths of red observed increase
The diffraction pattern of white light diffracted through a single slit
- A source of white light diffracted through a single slit will produce the following intensity pattern:
- The central maxima is equal in intensity to that of monochromatic light
- The non-central maxima are wider and less intense
- The fringe spacing between the maxima get smaller
- The amount of red wavelengths in the pattern increases with increasing maxima, n increases from n = 1, 2, 3...
- The amount of blue wavelengths decrease with increasing maxima
The intensity pattern for the diffraction of white light through a single slit