Shine a laser through a narrow slit and the light spreads out. It does not travel in a straight line as a beam. It bends around the edges of the slit and forms a pattern of bright and dark bands on a screen. That is diffraction, the spreading of waves around obstacles or through openings. It happens with all waves, including sound and water waves. It becomes noticeable when the size of the opening is comparable to the wavelength of the wave.

Diffraction is why you can hear someone talking around a corner. Sound waves have wavelengths of about a meter, and they bend around obstacles that size. Light waves have wavelengths of about 500 nanometers, so diffraction is only noticeable through very small openings. Diffraction limits the resolution of microscopes and telescopes. It is also the basis of diffraction gratings, which split light into its component colors. X-ray crystallography uses diffraction to determine the structure of crystals, including proteins and DNA. The famous Photo 51, which helped reveal the double helix structure of DNA, was an X-ray diffraction image.

Diffraction facts

• Definition. Spreading of waves around obstacles.
• Condition. Noticeable when opening size is close to wavelength.
• Sound. Bends around corners because of long wavelengths.
• Light. Bends through very small openings.
• Applications. Diffraction gratings, X-ray crystallography.

Diffraction is not the same as refraction. Diffraction is bending around obstacles. Refraction is bending as light passes between media.