If  a beam of light is passed through a wide opening, it passes through with little or no diffraction. However, if the opening is small, the wave property of light often produce unusually looking pattern - consisting of bright fringe (called central maximum), the dark fringes  and the less-intense bright fringes (called the secondary maxima)
Teacher reviews Diffraction of waves with the class by asking probing questions like explanation of how diffraction occurs.

Teacher discusses single-slit diffraction and
the conditions that should exist for it to occur.
Fraunhofer diffraction is discussed as a special case of diffraction that shows distinctive
differences between the central fringe and darker flanking fringes.

Teacher uses appropriate problem to show how to determine the width of the central maximum of
Fraunhofer’s diffraction.



Fig: Image of Single-slit  diffraction

   Fig: Diffraction of red laser beam on the hole
Source: http://en.wikipedia.org/wiki/Diffraction