CHEM 121 glossary
Diffraction
The spreading of a wave as it passes through a gap or around an obstacle, producing a pattern of bright and dark regions. It is strong only when the wavelength is comparable to the size of the gap or the spacing of the obstacles.
Example
Davisson and Germer (1927) diffracted 54 eV electrons ( nm) off a nickel crystal, whose rows of atoms are a similar distance apart: direct evidence that electrons are waves.