Any angle of incidence above this will totally internally reflect. This is used both as a communication system, and as a sort of camera in hard to reach places. Short wavelength light will be refracted more, and long wave length less. This means that if white light is shone onto a prism, then the light can be separated out into it's component colors, red being refracted the least, and violet the most. This allows the interference pattern, and the amplitude at each point to be found.
Path difference is the difference between the distances of a certain point from each source.
Light strikes the two slits, and then produces two coherent point sources next to each other. As we move further around, the path difference will be 1 wavelength, they will reinforce, and produce a bright band, and so on alternating. This means the gap between the two pieces of glass increases going further out from the center. These slits produce much more precise lines, because rather than just requiring two beams to coincide, they require many to do so. This produces a much sharper pattern, and is more easy to analyze.