⇒ When waves pass through a gap or move past an obstacle, they spread out (i.e. diffract)
⇒ Diffraction happens when waves spread around an obstacle or through a gap
⇒ Diffraction is most pronounced if it is of a similar order of magnitude (i.e. similar size) as the obstacle or gap
⇒ Visible light has an extremely short wavelength so diffraction is only significant if the slit is very narrow
⇒ When light diffracts though a single slit, a fringe pattern on a screen can be seen with a central bright fringe and less bright fringes each side
⇒ These bright fringes (called maxima) are due to constructive interference
⇒ The dark fringes in between (minima) are due to destructive inteference
⇒ The width of the central maximum depends on the wavelength of the light and the slit width
⇒ As the wavelength of light increases, the central maximum becomes wider.
⇒ As the slit width increases, the central maximum becomes narrower
⇒ If white light is used and diffracted through a single slit, the fringes are coloured because white light contains a mix of colours
⇒ Interference patterns can be seen using diffraction grating, which has thousands of slits spaced very closely together
⇒ As the slits are so narrow, the light diffracts at a wide angle
⇒ The resulting pattern is due to the light superposing and interfering from the slits
⇒ For the first order maxima:
⇒ Applying this information to the triangle abc above:
⇒ In general, the conditon for maximuma to occur is given by:
⇒ They are used to separate light of different wavelengths at great detail (i.e. in a high resolution)
⇒ This can be used as part of a spectrometer, which is used to investigate atomic spectra
⇒ It can also be used in telescopes to analyse light from another galaxy
⇒ Also see our notes on: