Diffraction Practicals

Diffraction effects can be demonstrated in many ways and practical methods for measuring (various) quantities are numerous. The intention here is to include at least two of these methods, however students may only need to do one. 
Lasers are commonly used for these practicals and even cheap ones can produce very satisfactory results, however care must be taken. Only class 1 and class 2 lasers should be used in school labs (not 1M, 1C, 2M or any class 3 / 4 lasers).