Four Chapter 12

Other Interference Effects

Resolution

• Diffraction is important in the design of microscopes and telescopes.
• Resolution - The ability to separate two closely spaced images.
  • Two stars when seen through a telescope have their light diffract. If the patterns overlap, an interference pattern forms which makes images fuzzy.
• With lenses, the smaller the lens, the more diffraction, therefore, the less it is able to resolve two different objects.
  • Size of the objective lens in a microscope limits the power of the microscope (Most microscopes are limited to about 600X).
  • This problem in telescopes is solved by using large lenses or mirrors.

Diffraction Gratings

• Diffraction grating - a device for wave analysis having a large number of equally spaced parallel slits.
  • Each slit acts as a line source of light.
  • Waves passing through several slits interfere
• The equations learned for double source interference apply to diffraction gratings.

• Differences between multislit and double slit interference patterns are:
  • Multislit is brighter (more space for light to pass through).
  • Multislit has sharper and narrower maxima.
  • Separation between successive maxima is greater with a grating.
• There are two types of gratings.
  • Transmission - light passes through the spaces between lines etched on glass or plastic.
  • reflection - light reflects off of spaces between lines etched on glass, plastic or metal.
• The amount of diffraction is determined by the ratio of wavelength to opening size. Therefore, each color of light (frequency or wavelength) will diffract through a different angle. This can be seen from the equation:

• With white light, the central maximum will be a sharp white peak.
• The other maxima will have each color diffract and produce a maximum at a different place.
• The result is a continuous spectrum on the first antinodal line and again on the second antinodal line and so on.
• In a spectroscope, light from the source moves in order
  • Through a collimator - a device that uses a system of lenses to ensure that the light rays are all parallel.
  • Through a diffraction grating
  • Through a telescope to view the interference pattern

Interference in Thin Films

• Thin films can produce interference patterns
  • Light entering a film will have some light striking the top surface of the film reflect.
  • Some light will refract as it passes through, and reflect off of the bottom surface of the film.
  • Further to this, if the light is incident on a surface that is more optically dense, then any reflected light will be inverted.
  • The two reflected rays travel different distances, and one may have been inverted and the other not, and therefore they are not necessarily in phase any longer.
  • This results in interference.
  • Whether the interference is constructive or not depends upon the relationship between path difference, whether the wave was inverted or not, and the wavelength of the wave.
  • Since different colors have different wavelengths, they produce constructive interference at different places.
  • Films are not of uniform thickness. This as well as the previous discussion, causes the color patterns seen in thin films like soap bubbles.
• Thin films of oil on water or air wedges between glass plates also produce interference in similar ways.
• Application of thin film interference
  • Checking the curvature of ground optics.

• Checking to see if ground metal is flat

• Coatings on lenses. The thickness of the coating is chosen so that reflection off of the lens surface interferes destructively with reflection off of the coating surface.

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Movember 21, 2013