At the start of this lesson you were asked the following essential questions:

  • What is diffraction?
  • What is an interference pattern?
  • What is the significance of Thomas Young's experiment as it relates to the wave model of light?
  • How are diffraction patterns solved mathematically in ideal and experimental conditions?

In this lesson you learned that diffraction occurs when any wave front bends or changes direction as it passes by the sharp edge of an obstacle or through a small opening in the obstacle.

 

If light is shone through two small openings (a double slit), the diffracted waves form an interference pattern characterized by a repeating pattern of constructive and destructive interference, which is explained by assuming light has wave-like characteristics.  For this reason, Thomas Young's double-slit experiment and observed interference pattern provided supporting evidence for the wave model of light.

 

You also learned that the interference pattern is described and analyzed as follows:

  • On either side of the central antinode, or bright fringe, in the centre of the pattern, there is a dark fringe, then a bright fringe, then a dark fringe, then a bright fringe, and so on.  Each fringe is given an order.

  • Double-slit diffraction patterns can be analyzed mathematically with the following two equations.  The first equation is valid for a very small angle θ .

         

  • The bright fringes, or antinodes, are the location of constructive interference where the path difference for the light travelling from both slits is a whole number of wavelengths, where n = 1, 2, 3, 4, ....

  • The dark fringes, or nodes, are the location of destructive interference where the path difference is offset by a half-number of wavelengths, where n = 0.5, 1.5, 2.5, 3.5, ....

You also learned that diffraction gratings have a large number of equally spaced, parallel lines.  The equations that describe a two-slit experiment are equally useful for gratings since they only rely on the spacing between any two slits, or grooves, such as those making up the track on a CD or DVD.

 

Assignment 12

Please complete Assignment 12.   When you are finished, upload your document to the Assignment 12 page.  Move to the next page to find the Assignment 12 page.