Two-slit experiment and

K = I, II. The situation is quite analogous to the usual discussion25 of the two slit experiment , and just as there, it is not proper to say that the n, - n2 transition takes place via trajectory I or trajectory II, for logic would then demand that the probabilities add rather there is a probability amplitude for the trajectory being I or II, and these amplitudes add.25... 

Fraunhofer was also the inventor of diffraction grating, a series of parallel wires separated by variable distances as small as 0.005 cm, which acted like an enhanced version of Young s two-slit experiment and could also be used as a replacement for the prism. Passing light through the spaces between the wires forms an interference pattern beyond the grating, and the direction of constructive interference is different for each wavelength. 

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