Light and X rays as wave motions

FIGURE 3.1. The amplitude A, wavelength A, and relative phase t (relative to a chosen origin) of a sinusoidal wave. The vertical axis represents the extent of deviation from a base line. The horizontal axis, and hence the phase, represents the time (or distance) required for the wave crest to reach a defined point. 

1 Diffraction of visible light by small holes and slits 

Interference between waves occurs when they are traveling in the same direction. They do not appreciably affect each other when they are traveling in different directions. When his son asked Can waves cross  

Sir Lawrence Bragg quickly replied/ Of course they can, otherwise you could not see your brother across the table, while I was looking at your mother.  

So far we have only considered the diffraction pattern of a single slit and have shown that the intensity variation is bell shaped this is the envelope profile with a width inversely proportional to the width of the slit. Now we will consider what happens to the diffraction pattern when more slits are lined up parallel to the first to give the equivalent of a diffraction grating. This is a two-dimensional analogy to the buildup of a crystal 

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