A16 White light through slits

What happens if we pass a beam of white light through a double slit?

pic1

For reference, the interference pattern formed by passing monochromatic green light (l=500 nm) through the double-slit is included. You can imagine the fringes formed by monochromatic red light (l=700 nm) are broader and more spaced out, and those formed by monochromatic violet light (l=400 nm) are narrower and closer together.

White light consists of light waves of wavelength between 400 nm to 700 nm. So you can think of the the resulting interference pattern as the summation of all the fringes of all the colors.

  • All the different color bands are aligned at the centre, forming the central white band (with a reddish tint at the edge because only the longest wavelengths extend further out).
  • If you keep in mind that the higher order fringes of all colours are slightly misaligned, you can make sense of appearance of the higher order bands.

If we add more slits, we can make the fringes narrower. Shown here is the pattern formed by a 15-slit grating.

pic2

Again, for reference, the interference pattern formed by passing monochromatic green light (l=500 nm) through the double-slit is included. The fringes formed by monochromatic red light (l=700 nm) would be further out, while those formed by monochromatic violet light (l=400 nm) would be closer to the centre.

  • The central white line is simply the summation of all the 0th order fringe of all the colours.
  • On either side, we see a distinct 1st order spectrum, formed by the 1st order fringe of all the colours, beginning with violet, ending with red.
  • Even further out, we have the 2nd order spectrum, formed by the 2nd order fringe of all the colours,
  • If the slit separation were larger, we can have the (n+1)th order spectrum starting (the violet end) before the nth order spectrum ending (the red end), resulting in overlapping spectrums.

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