Use(s) Material is not Suited For: Tutorial, Self-Study, Detailed analysis of interference patterns.
Units are not used for slit distance or wavelength. There should be discussion about the importance of the ratio of these quantities.
The path difference is negative when examining a point in the upper half of the screen. This is contrary to typical sign conventions (lower path minus upper) that are used to make path difference positive when angle is usually considered positive.
The lack of accurate description and lack of quantitative analysis of intensity may limit the uses for student self-study.
General Comments on Effectiveness: The applet is visually effective, with regions of maximum interference clearly visible. However these constructive interference regions appear white (for high average intensity regions), but the peaks or wavecrests are displayed as black. The effect is very visible for small wavelength.
One can easily determine conditions such that the path difference is an integral number of wavelengths using the red triangular markers. An improvement would be to allow for switching to a view that displays a gray scale representing intensity at each point (or time average intensity).