Paper 2 · Waves and Interference
Why is diffraction of sound waves a common, noticeable phenomenon in everyday life, while diffraction of light waves is not?
ALight waves do not diffract at all under any circumstances, because their electric and magnetic field components are assumed here to force them to always travel in perfectly straight lines, with no bending around any obstacle or through any gap however narrow.
BSound travels more slowly than light, and only slower-moving waves are able to diffract around an obstacle.
CSound waves are transverse while light waves are longitudinal, and only longitudinal waves diffract to a noticeable extent.
DSound wavelengths (millimetres to several metres) are comparable to the size of everyday gaps and obstacles, so sound diffracts noticeably; light's much shorter wavelength (about 5×10^-7 m) needs an extremely narrow slit to show noticeable diffraction.
Explanation: Sound wavelengths are comparable in size to everyday gaps and obstacles, so sound diffracts noticeably around them. Light's much shorter wavelength (~5×10^-7 m) needs an extremely narrow slit to show noticeable diffraction, so under normal conditions light appears to travel in straight lines.
Derived from ZIMSEC Physics Paper 2, November 2002, Q4