Drop a pebble into a still pond and rings spread outward. A leaf floating nearby bobs up and down as they pass, but it does not travel along with them. That simple scene holds the key idea of this level: a wave carries energy from place to place, while the material it passes through mostly stays where it is. Sound, light, earthquakes and the ripples on a pond are all waves, and the same few ideas describe them all.

Describing a wave

Three terms describe almost any wave. Wavelength is the distance from one crest to the next. Frequency is the number of waves passing a point each second, measured in hertz. Amplitude is the height of the wave, which sets how much energy it carries. For waves moving at a fixed speed, a longer wavelength means a lower frequency. Waves come in two main types. In transverse waves the medium moves at right angles to the direction the wave travels, like a rope flicked up and down. In longitudinal waves the medium moves back and forth along the direction of travel, like a pushed spring.

Sound

Sound is a longitudinal pressure wave, a series of squeezes and stretches passing through a material such as air, water or rock. Because it needs a material to travel through, sound cannot cross a vacuum, and battles in outer space should really be silent. In air at room temperature sound moves at about 343 meters per second, roughly a kilometer in three seconds. It travels about four times faster in water and faster still in steel.

The pitch of a sound depends on its frequency, and its loudness on its amplitude. Young people hear from roughly 20 to 20,000 hertz, and the upper limit drops with age; dogs and bats hear much higher.

When a source of waves moves, the waves bunch up ahead of it and stretch out behind. This is the Doppler effect: a passing ambulance's siren sounds higher as it approaches and lower as it moves away. The Austrian physicist Christian Doppler proposed the idea in 1842. In 1845 the Dutch scientist C. H. D. Buys Ballot tested it in a memorable way: trumpeters played a steady note on a moving train while musicians beside the track judged the pitch.

When two waves meet, they add together. Crest on crest makes a bigger wave, and crest on trough cancels out. This is called interference. Noise-cancelling headphones use interference: a microphone picks up outside noise, and a speaker plays the opposite wave so that the two largely cancel. A related effect is resonance, which happens when something is pushed at its own natural frequency, so that small pushes build into large vibrations, as when a child on a swing is pushed in rhythm.

Light

The last level ended with Maxwell's finding that light is an electromagnetic wave. But for two centuries before that, scientists had argued over whether light was a wave or a stream of tiny particles. Newton favored particles, and his authority carried weight. In the early 1800s the English physician and scholar Thomas Young shone light through two narrow slits and saw a pattern of bright and dark bands on a screen. Young's double slit experiment showed that light interferes like a wave, with bright bands where waves from the two slits reinforce each other and dark bands where they cancel.

Visible light is a narrow slice of the electromagnetic spectrum. Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves that differ only in wavelength. Radio waves can be longer than a building; gamma rays are smaller than an atom. Within the visible band, red has the longest wavelength and violet the shortest. Light slows down when it passes from air into glass or water, and this makes it bend, an effect called refraction. Lenses in eyeglasses and cameras rely on it. Because different colors bend by slightly different amounts, a prism or a raindrop spreads sunlight into a rainbow. Newton showed this with prisms in the 1660s, and drew the lesson that white light is a mixture of all the colors of the visible spectrum.

Worked example: why the sky is blue

Sunlight contains all colors, yet the daytime sky is blue. The reason is scattering. As sunlight passes through the air, the tiny molecules of nitrogen and oxygen scatter some of it in all directions, and they scatter short wavelengths far more strongly than long ones. Lord Rayleigh worked out the rule in 1871. Blue light is scattered across the whole sky, so blue reaches your eye from every direction you look. Violet is scattered even more, but sunlight contains less of it and our eyes are less sensitive to it.

At sunset, sunlight must cross far more air to reach you. Most of the blue is scattered out of the direct beam before it arrives, leaving the reds and oranges that color the evening clouds. The same physics explains why distant mountains look bluish, and why on the Moon, which has no air to scatter light, the sky stays black even in full sunshine. A question a child might ask turns out to be answered by wavelength, scattering and the makeup of the atmosphere.