White Light Diffraction Grating

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White Light Diffraction Grating

White Light Is Incident On A Diffraction Grating As Shown In Fig 4.1

White Light Diffraction GratingWhite

Diffraction and Interference

Grating

A different sort of spectrometer component called an immersed grating also consists of a prism with a diffraction grating ruled on one surface. However, in this case the grating is used in reflection, with light hitting the grating from inside the prism before being totally internally reflected back into the prism (and leaving from a different. A diffraction grating is a large number of evenly spaced parallel slits. (a) Light passing through is diffracted in a pattern similar to a double slit, with bright regions at various angles. (b) The pattern obtained for white light incident on a grating.

posted Jul 2, 2012, 12:53 PM by Unknown user [ updated Jul 2, 2012, 2:56 PM]

It shows that for a given diffraction grating (at fixed b), a different wavelength gives maxima at different points of the spectrum. This is the basis of the use of diffraction gratings in optical spectroscopy. If white light falls on a diffraction grating each of the main maxima is broadened. A diffraction grating is often (but not only) used to split the white light into colours. It is also used to determine the wavelength of a light wave similarly to the methods mentioned earlier in single-slit diffraction and Young Double-Slit Experiment.

White Light Through Diffraction Grating

Light

What Is A Diffraction Grating

In this topic we look at how waves diffract when they pass through a slit in a barrier, Young's double slit experiment, diffraction gratings and white light spectra.
Diffraction:
  • Waves bend at the edges when they pass through a gap in a barrier. This is known as diffraction.
  • The smaller the gap the greater the bending of the wave.
  • If the gap is equal to or smaller than one
  • wavelength, then parallel circular waves emerge.
  • The wavelength, frequency and speed of diffracted waves are unaffected.
Interference:
Interference patterns can be observed using microwaves.
  • Two sets of coherent waves can be created by passing one wave through a barrier with two slits.

  • At point X two waves have traveled different distances to create a maxima.
  • One wave has traveled S1X and the other S2X. The difference between these two distances, S2X-S1X, is known as the path difference.
  • To create maxima the waves must be in phase and so the path difference must be a whole number of wavelengths.
  • For minima, destructive interference must occur. For the waves to be completely out of phase, one wave will have traveled an odd number of half wavelengths more than the other.
Path difference = (n+1/2)λ
Explained further in the 'Young's Slits Experiment' page.
The Diffraction Grating:
  • The interference pattern produced using a double slit is not very bright as very little energy is passed through the narrow slits.
  • A diffraction grating has hundreds of lines per millimeter meaning a much brighter interference pattern is produced.
  • The first position from the central position where constructive interference occurs is known as the first order maxima.
  • These two positions are found at equal angles either side of the central maximum.
  • The relationship between this viewing angle (θ), the slit spacing (d), the wavelength (λ) and the order of maximum is:

  • The range of wavelengths in light is known as a spectrum.
  • Since visible light is a mixture of red, green and blue wavelengths (and colours) it is often referred to as white light.
  • As with the Young's Slits Experiment, different wavelengths produce lines of different separation in diffraction gratings.
  • When white light is passed through a diffraction grating coloured spectra are seen either side of the central maxima as the path difference depends on the wavelength of the light.
  • A spectrum is also produced when white light is passed through a prism.
For a prism:
  1. (order of colours, deviated least to most) red, orange, yellow, green, blue, indigo, violet.
  2. there is no white central maximum.
  3. only one spectrum.
  4. spectrum produced by refraction.

For a diffraction grating:
  1. (order of colours, deviated least to most) violet, indigo, blue, green, yellow, orange, red.
  2. white central maximum.
  3. many spectra in pairs either side of the central maximum.
  4. spectrum produced by diffraction.