Jugami › Physical Science › Waves
Waves carry energy from place to place without carrying the stuff along.
Music from a speaker, light from the Sun, the signal that reaches your phone, and the waves rolling onto a beach are all waves. Why do you see lightning before you hear thunder? How does a bat find bugs in the dark? On this page you'll learn the parts of a wave, how to find a wave's speed, and how sound and light behave when they hit something.
A wave is a disturbance that carries energy from one place to another. The wave moves along, but the stuff it travels through mostly just wiggles in place and stays where it was.
Watch a duck on a pond when a ripple passes: it bobs up and down, but it isn't carried away to shore. In a stadium "wave," fans stand up and sit down in their seats, yet the wave races around the whole stadium.
Mechanical waves need a medium, some matter to travel through, like water, air, or a rope. Ocean waves, sound, and earthquake waves are mechanical.
Electromagnetic waves don't need a medium. They can travel through empty space, which is how sunlight reaches Earth. Light, radio, and microwaves are electromagnetic waves.
Waves also differ in how the medium moves:
The speed of a wave equals its frequency times its wavelength:
v = f × λ
With f in hertz and λ in meters, the speed v comes out in meters per second (m/s). In one medium, waves of one kind travel at about the same speed, so if the frequency goes up, the wavelength gets shorter.
Worked examples (made-up numbers, except the speed of sound)
Ocean waves: 0.5 waves pass a pier each second (f = 0.5 Hz), and the crests are 8 m apart. v = 0.5 Hz × 8 m = 4 m/s.
A rope wave: you shake a rope 3 times a second, and the crests are 0.6 m apart. v = 3 Hz × 0.6 m = 1.8 m/s.
Working backward: sound travels about 343 m/s in air at 20 °C. A note with a frequency of 343 Hz has λ = v ÷ f = 343 ÷ 343 = 1 m.
This make-believe rope carries waves at a fixed speed of 1 m/s. Change the amplitude and frequency and watch the wavelength.
Wavelength = 1 m/s ÷ 1.0 Hz = 1.00 m
The box shows 3 meters of rope.
Sound is a longitudinal mechanical wave made by something vibrating, like a guitar string, a drum skin, or your vocal cords. Because it needs a medium, sound can't travel through the near-empty space between planets.
In air at 20 °C, sound travels about 343 m/s. It moves faster in water (about 1,500 m/s) and faster still in many solids, such as steel. Light is far faster, which is why you see lightning before you hear thunder: the sound takes about 3 seconds to travel 1 kilometer.
Light is an electromagnetic wave. In a vacuum, all electromagnetic waves travel at the same speed: about 300,000 km/s (299,792 km/s). Sunlight takes about 8 minutes to reach Earth.
The full range of electromagnetic waves is the electromagnetic spectrum. From longest wavelength (lowest frequency) to shortest (highest frequency):
Visible light is a mix of colors, from red (longest wavelength) through orange, yellow, green, and blue to violet (shortest). A rainbow forms when raindrops bend and spread sunlight into those colors.
Ask an adult before you start. Wipe up spills right away, and be gentle with glass.
You need: a slinky or a long rope, a partner, a smooth floor.
Stretch the slinky along the floor with your partner holding the other end. Flick your end sideways to make a transverse wave. Then push your end forward and back to make a longitudinal wave. Try slow and fast shakes.
Why: Faster shaking means higher frequency, so the waves get shorter. The coils stay in place while the energy travels.
You need: 4–6 matching glasses or jars, water, a metal spoon.
Fill the glasses with different amounts of water. Tap each one gently with the spoon and put them in order from low note to high.
Why: More water makes the glass vibrate more slowly when tapped, so it gives a lower pitch (lower frequency).
You need: a clear glass of water, a straw or pencil.
Put the straw in the water and look at it from the side, level with the water's surface. Then look from straight above.
Why: Light from the underwater part changes speed and bends as it leaves the water, so the straw seems to break: refraction.
You need: two paper cups, a few meters of string, a pencil, a partner.
Have an adult poke a small hole in the bottom of each cup with the pencil, thread the string through, and knot it inside. Pull the string tight and take turns talking and listening.
Why: Your voice makes the cup vibrate, and the vibrations travel along the tight string. Pinch the string and the sound stops.
Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.
