Diffraction Gratings, and Interference in Thin Films

1
Diffraction Gratings,and Interference in Thin
Films

• From soap bubbles to solar panels

2
What are diffraction gratings?

• So far, we have discussed and learned about
  single and double slit interference. What happens
  when there are multiple slits?
• Diffraction gratings are devices that have large
  numbers of equally spaced parallel slits that act
  as individual light sources. The diffraction
  patterns produced by these devices resemble those
  produced by double slits. As such, their minima
  and maxima can be calculated using

3
About Contd

• So, what are some of the differences between
  double slit and a multiple slit interference
  patterns?
• Multi-slit sources deliver more light energy
  i.e. higher intensity. This produces brighter
  patterns.
• Multi-slit sources have much sharper and narrower
  maxima.
• The distance between successive maximums are
  greater on account of smaller slit separations,
  enhancing the resolution.

4
Types of diffraction gratings

• There are two common types of gratings
• Transmission The spaces between the diffraction
  lines (opaque) transmit light.
• Reflected The spaces between the gratings
  reflect the light as opposed to transmitting it.
  Metals can also be used.
• Another type, although not
• as common, is the crossed
• grating. The image to the
• left is a pattern produced by
• such a grating

5
Application Spectroscope

• A well known example of an application of a
  diffraction grating is the spectroscope.
  Spectroscopes are used perform spectral analysis.
  
• Recall that with a diffraction grating, the
  central maximum was mainly white light. However,
  as we went further out from the central maximum,
  we had a more pronounced distribution of colours.
  
• Why do you think this is so?

6
Tomorrow Interference in Thin films

• What do soap bubbles and solar panels have in
  common?