In this exercise you explore the regime of Fresnel diffraction with
a circular aperture. The incident light is assumed to be a plane
wave. You can change the diameter of the aperture, the wavelength
of the light (and thus the ratio of the two), and the distance from the
aperture to the plane of observation. The program will calculate
the Fresnel number for the parameters you have chosen, so you can compare
the effects of having even and odd numbers of Fresnel zones contained in
the aperture. The incident light is assumed to be a plane wave.
You boot the program up in the usual way by following this link to
WEBTOP .
When you change the parameter settings, you will need to hit "enter" to
make the program respond.
1. With the default settings, is the spot in the center bright
or dark? Do you understand why?
2. Now move the screen to various positions and watch the intensity
in the center change. How is this correlated with changes in the Fresnel
number?
3. Change the aperture size and the wavelength but keep their
ratio constant. What happens? Why?
4. Now just play around and see if you can predict (or at least
postdict) what you see as you change the parameters.