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
- The features of the single slit diffraction pattern are:
- A central maximum with a high intensity
- Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum
- 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 colours look blurry and further away from the central maxima 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 colour observed decrease and the wavelengths of red observed increase
- The fringe spacing would be smaller and the non-central maxima would be wider
Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width