WEBTOP Exercise

Fraunhofer N-slit

In this exercise you explore the combined interference and diffraction pattern generated in the far field by parallel slits. The parameters you can vary (shown at the bottom of the WEBTOP screen) are the number of slits, the wavelength of the light, the width of the slits, the distance (center-to-center) between slits, and the distance from the slit plane to the observation plane. As always, when you change a parameter, you must hit "enter" to have it take effect. You get to WEBTOP here.

Starting with the default settings, notice the characteristics of the pattern. These include the number of secondary maxima between principal maxima, the number of principal interference maxima within the central diffraction band, the height of the central maximum, and the number of principal maxima contained within the 20-mm width shown on the observation plane. Are these what you would calculate from the expression you derive via Fourier transform of the transmission function, when you put in the appropriate distance scaling parameters?
Now change the parameters one by one. What happens when the slit spacing is an integral multiple of the slit width? When the wavelength is an integral multiple of the slit width?
Move the observation screen to a position where you can see the parts of the pattern outside the central maximum. How does the intensity of that portion of the pattern change as you vary the parameters? Does this make sense?