Diffraction
The way a wave bends around the edge of an obstacle or spreads out after passing through a narrow gap. Where the waves overlap they reinforce or cancel, producing bright and dark bands.
Water waves passing through a gap in a breakwater fan out. Light does the same, and bright and dark bands appear at the edges of the shadows of thin objects. In 1803 the English physicist Thomas Young showed that such bands can be explained by two light waves overlapping and reinforcing or cancelling each other (interference), which strongly supported the view that light is a wave[1].
Diffraction and interference are things only waves can do, so they settle whether something is a wave. In 1927 Clinton Davisson and Lester Germer in the United States, with a nickel crystal[2], and G. P. Thomson and Alexander Reid in Britain, with a thin film[3], caught electrons making diffraction patterns, confirming that electrons have a wave nature too.
The diffraction patterns of X-rays or electrons scattered by a crystal reveal how its atoms are arranged. It remains a standard way of working out the structure of matter.
Related terms
- Matter waveThe idea that things long taken to be particles, such as electrons, also behave as waves. Proposed by Louis de Broglie in 1924.
- Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
- SpectrumLight spread out by wavelength (colour). Atoms and molecules each emit and absorb only certain wavelengths, so thin lines appear in the spectrum.
Sources
- T. Young, “The Bakerian Lecture: Experiments and calculations relative to physical optics,” Philos. Trans. R. Soc. Lond. 94, 1–16 (1804).
- C. Davisson, L. H. Germer, “The Scattering of Electrons by a Single Crystal of Nickel,” Nature 119, 558–560 (1927).
- G. P. Thomson, A. Reid, “Diffraction of Cathode Rays by a Thin Film,” Nature 119, 890 (1927).