Waves Quiz: Test Your Physics Knowledge

Understanding waves: essential concepts

A wave is the propagation of a disturbance through a medium or vacuum, without overall transport of matter. There are two main families: mechanical waves, such as sound or a wave on a string, which require a medium, and electromagnetic waves, such as light, which can propagate in a vacuum. A wave is described in particular by its amplitude, its frequency (in hertz), its wavelength λ and its propagation speed. These quantities are linked by the fundamental relation λ = v / T, or λ = v / f, where T is the period and f the frequency.

A very common misconception is to confuse frequency and speed. When a wave passes from one medium to another (for example from air to water), its frequency remains unchanged: it is the speed and therefore the wavelength that change. Another frequent error is to believe that sound travels faster in air than in a solid: in reality, sound propagates faster in steel (about 5000 m/s) than in air (about 340 m/s at 20 °C), because the medium is stiffer. Finally, one should not confuse transverse waves, where the disturbance is perpendicular to the direction of propagation (light, wave on a string), and longitudinal waves, where it is parallel (sound).

The phenomena of superposition deserve particular attention. When two waves meet, the resulting disturbance is the sum of the individual disturbances: this is superposition. Constructive interference occurs when the waves are in phase (path difference equal to a whole number of wavelengths), and destructive interference when they are in phase opposition (path difference equal to an odd number of half-wavelengths). Diffraction becomes significant when the size of the aperture is of the same order of magnitude as the wavelength, which explains why sound bends around a door while light, with its much shorter wavelength, seems to propagate in a straight line. Mastering these notions makes it much easier to solve typical physics exercises.