Light & Waves

What this part of the map covers

Mechanical waves and electromagnetic waves are two important ways that energy moves around the world.^[5] Water waves and sound are mechanical waves; light is electromagnetic.^[5] This page covers both kinds.

What a wave is

A mechanical wave forms when energy is passed through a medium, such as water or air.^[18] In a transverse wave, the vibration is at right angles to the direction the wave travels, as on a vibrating string; in a longitudinal wave, it is along that direction.^{[23, 20]}

The distance from one crest of a wave to the next is its wavelength.^{[5, 19]} The number of crests passing a point each second is its frequency, measured in hertz (Hz): one hertz is one cycle per second.^{[5, 17]} The longer the wavelength, the lower the frequency.^{[19, 16]}

Sound

Sound waves are longitudinal: they travel by alternately squeezing (compression) and stretching (rarefaction) the air or water they pass through.^{[17, 18]} Because sound needs a medium, it cannot travel through the vacuum of space.^[5] The higher the pitch, the higher the frequency, and loudness goes with the wave’s amplitude, or intensity.^{[17, 18]}

The speed of sound depends on the air or gas it travels through, not on what made the sound, and in air it rises with temperature.^{[16, 17]} At sea level and 15 °C, sound travels at about 340 metres per second.^[17] When a sound source moves, its waves bunch up in front of it and spread out behind it.^[16] This Doppler shift happens with any kind of wave, light included.^[15]

Light and the electromagnetic spectrum

The words light, electromagnetic waves and radiation all name the same thing: electromagnetic energy.^[5] Unlike sound, electromagnetic waves need no medium, so they can cross the vacuum of space.^{[5, 19]} Moving along the spectrum from long to short wavelengths, the energy increases.^[5]

• Radio waves have the longest wavelengths, from the length of a football to larger than our planet.^[7]
• Microwaves can get through haze, light rain, snow, clouds and smoke, which makes them useful for satellite communication.^[8]
• Infrared light is invisible to the eye, but we can feel it as heat.^[9]
• Visible light is typically the wavelengths from 380 to 700 nanometres (billionths of a metre).^[10] The US Geological Survey (USGS) gives 0.4 to 0.7 micrometres.^[19]
• Ultraviolet light has shorter wavelengths than visible light; we can’t see it, but bumblebees can.^[11]
• X-rays have wavelengths between 0.03 and 3 nanometres.^[12]
• Gamma rays have the shortest wavelengths and the most energy.^[13]

The speed of light

In a vacuum, light travels at 299,792,458 metres per second.^{[2, 1]} That number is exact: since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second.^{[3, 2]} Light travels more slowly in air than in a vacuum, and more slowly still in water.^{[6, 27]}

Reflection, refraction and colour

For any electromagnetic wave, the angle at which it strikes a surface equals the angle at which it bounces off: the law of reflection.^[24] Refraction is the change of direction when light passes into a material with a different refractive index, a number comparing light’s speed in the material with its speed in a vacuum.^{[25, 6]} It is why an object partly under water looks distorted, and it lets a lens focus light, as in eyeglasses and magnifying glasses.^{[25, 26]}

The refractive index changes with wavelength, an effect called dispersion.^[25] A prism separates white light into the colours of the rainbow because each colour is a different wavelength, from violet, the shortest, to red, the longest.^{[10, 19]} An object’s colour is the wavelengths it reflects; the others are absorbed.^{[6, 24]}

Diffraction and interference

Diffraction is the bending and spreading of waves around an obstacle, strongest when the obstacle is about as big as the wavelength.^[6] Overlapping sound waves can reinforce or interfere with each other.^[17] In the early 1800s Thomas Young shone light through two small, closely spaced openings and saw bright bands separated by dark regions.^{[22, 27]} Only a wave could make such a pattern.^[22]

Yet light is also made of packets of energy called photons, and all light has both wave-like and particle-like properties.^{[5, 22]} The Modern Physics page picks up this story.

The math (optional)

For sound, speed equals wavelength times frequency.^[17] For light, wavelength equals the speed of light divided by the frequency.^[14]

sound: speed = wavelength × frequency

Our own example: a sound with a wavelength of 1 metre, travelling at 340 metres per second, has a frequency of 340 Hz.^[17]

A short history

Ibn al-Haytham (Alhazen) gave the first correct explanation of vision, that light reflects from an object into the eye.^[28] Willebrord Snell found the law of refraction in 1621 but never published it; Descartes published it first, in 1637.^{[27, 29]}

In 1678 Christiaan Huygens argued for a wave theory of light.^[30] Isaac Newton concluded that light is a stream of particles, and published Opticks in 1704.^{[33, 34]} Young published his interference results in 1801, and the wave theory went on to win wide acceptance in the 19th century.^{[27, 34]}

Around 1862 James Clerk Maxwell calculated that electromagnetic fields travel at about the speed of light, and proposed that light is electromagnetic.^{[31, 27]} Heinrich Hertz proved radio waves exist in the late 1880s.^[7] He did not expect much of them: “I do not think that the wireless waves I have discovered will have any practical application.”^[32]

Common misconceptions

“The siren itself changes its note as it passes.” The change is in what a listener hears: for someone riding on the fire engine, the siren’s frequency stays the same.^[15]

“Light must be either a wave or a particle.” It shows both kinds of behaviour.^{[5, 22]}

Going further

Start with Energy & Work, then see Electricity & Magnetism, Heat & Thermodynamics and Space Telescopes. Lasers, the eye and quantum optics are left for deeper pages.