Light appears to bend {diffraction}| around corners and edges. If light rays meet corners, corner re-radiates light in all directions, so some light goes to region behind edge. Wavelets add to form wavefront there. At most wavefront points, wavelets cancel each other, so light intensity is zero. At some wavefront points, sum is positive, and light appears behind edge at regular intervals. Shadows have diffraction patterns at edges.
sound
Diffraction is how people can hear sound around corners.
size
If obstacle or edge is smaller than wavelength, wave goes farther around obstacle or edge. If obstacle or edge is larger than wavelength, diffraction has smaller angle.
frequency
Higher-frequency light and sound have smaller diffraction, because wavelengths are smaller. Lower-frequency light and sound bend more.
Materials {diffraction grating}| can have regular repeating opening or ruling patterns, so surfaces are like many edges. Diffraction gratings can be for parallel rays {Fraunhofer grating} or spherical rays {Fresnel grating}. The many edges cause strong diffraction pattern, because more wavelets add together to make higher amplitude. If openings are small or rulings have close spacing, diffraction is more, because smaller edge can re-radiate more behind edge.
Transparent plates {phase plate} with varying thickness can delay light slightly, to change phase. Phase plates are diffraction gratings. If only parallel light rays reach phase plate, diffraction is regular. Phase differences cause intensity differences at various points, by interference effects.
Shadows {shadow}| have umbra and penumbra.
Shadows have a lighter part {penumbra}|, where diffracted light enters.
Shadows have a dark part {umbra}|, where no diffracted light enters.
If light wavelength is less than object diameter, light bounces off object {scattering, light}|. If light wavelength is more than object diameter, light goes around object.
example
Sky is blue, because blue light has small enough wavelength to scatter from air molecules, but other colors have longer wavelengths. Air molecules are large enough to block blue and some green light from Sun, but longer wavelengths go around air molecules. Scattered blue light goes all over sky to make it blue instead of clear. Sun is red at horizon, because light goes through more atmosphere to eye, and air scatters blue, green, and yellow light.
X-rays can have elastic scattering {Compton scattering} from stationary electrons in light elements. Scattered-radiation frequency decreases with increasing angle, so high frequencies are at narrow angles.
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Date Modified: 2022.0225