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Law of Reflection

When light encounters a new material some can bounce back.

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Oil and Water

Oil and Water

Credit: Anton
Source: http://simple.wikipedia.org/wiki/File:Oelfleckerp.jpg
License: CC BY-NC 3.0

When oil and water are together, an interference pattern is produced. This is a result of the phase change of light upon reflecting from a medium with a larger index of refraction.

Amazing But True

  • Consider a thin film of oil and water. Part of the light that hits the film is reflected from the first interface, while the rest of the light is transmitted into the film. The light that is initially reflected under goes an 180° phase shift because it is reflecting off of a medium with a greater index of refraction. The light that is transmitted undergoes no phase shift and continues onto the next interface. When the light hits the next surface, the second set of reflected light goes through an 180° phase change.
  • This second reflected beam of light travels back up and out of the film where it will constructively or destructively combine with the light that is initially reflected off the top layer. Whether there is constructive or destructive interference, depends on the thickness of the films used.
  • Discover more about thin-film interference: http://www.tuks.nl/Mirror/labman.phys.utk.edu/Thin%20films.htm

Show What You’ve Learned

Using the information provided above, answer the following questions.

  1. If the phase difference between two waves is given as \begin{align*}\delta = \frac{R}{\lambda}(360^\circ)\end{align*} where \begin{align*}R\end{align*} is the path difference and \begin{align*}\lambda\end{align*} is the wavelength. What is the path difference required for a 500 nm wavelength of light to have a phase difference of 180°?
  2. If light originally traveling in a piece of glass hit a glass-air interface, does the reflected wave experience a phase change?
  3. How is the theory of thin film interference useful for creating sunglasses?

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Image Attributions

  1. [1]^ Credit: Anton; Source: http://simple.wikipedia.org/wiki/File:Oelfleckerp.jpg; License: CC BY-NC 3.0

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