Diffraction gratings and spectrographs

What creates the colors when light strikes the CD?Look at a compact disc illuminated by a room light. Do you see different colors or even a rainbow? What causes the different colors of light to bounce off the CD? The compact disc has many tiny grooves or rulings that encode the electronic files or music. When light strikes the surface, these grooves act similarly to a series of tiny slits that diffract and disperse the incident light. Read the text aloud
How diffraction gratings work
To understand how the CD dispersion works, imagine shining red light through two slits to produce the interference pattern on the previous page. As the number of slits increases, the individual peaks in the interference pattern become sharper. The reason for this sharper pattern is that, with many more slits, there are fewer places at which all the diffraction patterns line up to create constructive interference. This is a feature of diffraction gratings: As the number of slits increases, the lines become sharper. If the diffraction grating is illuminated with white light instead of red light, then a spectrum is formed at the location of each peak. This occurs because each different wavelength of light has its own location on the screen where constructive interference occurs. Read the text aloud
Diffraction through a novelty transmission gratingTransmission gratings—made commercially as novelties and toys, such as Rainbow Peepholes™ or Rainbow Glasses®—work by having many little rulings on thin plastic that act as a series of slits. If you look through one of these transmission gratings at a light source, such as a light bulb, you can see a series of rainbow spectra dispersed by the rulings. Read the text aloud
The CD acts as a reflection grating that disperses light by reflection instead of by allowing the light to pass through it. Reflection gratings are more effective than transmission gratings, so they are more commonly used for scientific research and commercial applications. A spectrograph is a scientific instrument that uses a diffraction grating to disperse light into its spectrum. Spectrographs can be small, hand-held devices, or they can be the size of a large truck when built for the largest telescopes on Earth. Read the text aloud
Dispersion through a transmission grating
  1. From the directions of the series of spectra, you can infer the alignment of the rulings—or of the different sets of rulings. Look at the illustration at right: Can you identify the orientation of all the different sets of rules?
  2. In the illustration you can also see a number of different orders of the diffraction grating. How many can you identify?
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