The simulation shows the effects on an interference pattern of varying

• The slit width.
• The slit separation
• The number of slits

The top screen shows a representation of the pattern that would be seen on a screen using a visible light source (laser) and the lower screen shows the pattern in terms of an intensity - angle graph.

N.B. The calculations have been done for slit widths of 1 to 10 wavelengths. In practice an actual slit would be many times wider than this which means that the interference pattern would be only a few degrees either side of the central part of the screen.

Look at the pattern obtained for a single slit. (This is usually referred to as a diffraction pattern). See how the pattern changes when the slit width is changed.

Now set the number of slits = 2 and the width = 1. See how the pattern changes as the slit separation is changed and also the width of the slits is changed. Notice that the maximum intensity increases - more light gets through two slits. The change in intensity is not shown in the graph since to do so would mean that the height of the maxima would be very small at low intensities.

The overall intensity is governed by the single slit diffraction curve shown in blue. In the Young's double slit experiment, you have slits that are many wavelengths wide and many wavelengths apart. Consequently what you see on the screen is the fringe pattern within a narrow central maximum.

Increase the number of slits keeping the width and the spacing constant and notice what happens to the intensity pattern.

In a diffraction grating there are typically around 10⁵ lines ("slits") per metre. This results in very intense, widely spaced principle maxima.