Unit 9, lesson 40. About 12 minutes.

What is a wave?

How does a wave cross the sea without carrying the water along with it?

Play

Getting the experiment ready…

Explain

Waves go up and down.

They carry energy along.

A wave carries energy from place to place, without carrying the stuff along with it.

The wavelength is the distance from one crest to the next. The amplitude is how tall the wave is.

The frequency is how many waves go past each second. It is measured in hertz (Hz).

v = f × λ. Wave speed equals frequency multiplied by wavelength.

Transverse waves, like those on a rope, vibrate at right angles to their direction. Longitudinal waves, like sound, vibrate along it.

Worked example: a wave has f = 2 Hz and λ = 3 m. So v = 2 × 3 = 6 m/s.

Something shakes
The shake travels
Energy moves, not stuff

Want the full story?

For a real rope, wave speed is set by the rope's tension and mass, so changing the frequency changes the wavelength instead. The sim lets you set both to show the formula. Ocean waves are more complex, and water moves in small circles.

The Formula

wave speed = frequency × wavelength

v = f \times \lambda

$v$: the wave speed, measured in metres per second (m/s)
$f$: the frequency, measured in hertz, or waves per second (Hz)
$\lambda$: the wavelength, written with the Greek letter lambda, measured in metres (m)

Try the numbers

Frequency: 1 HzWavelength: 2 m

Wave speed: 2 m/s

Try it at home

Ask a grown-up to help

You need a skipping rope or long string, a friend.

You need: a skipping rope or long string, a friend.

Shake a rope. See the wave go!

Tie one end of a rope to something firm, or have a friend hold it.
Flick your end once.
Now shake it steadily, faster and slower.

What's happening: the wave moves along the rope, but the rope itself stays put. Shaking faster shortens the wavelength.

Physics is life

Your body

Your ears catch waves.

Your ears catch sound waves, and your eyes catch light waves.

Daily life

Phones use invisible waves.

Wi-Fi, radio and phones all use invisible waves.

Nature

Earthquakes make waves.

Earthquakes send waves through the ground, and scientists use them to study the inside of Earth.

Space

Starlight is made of waves.

Light waves from distant stars have crossed space for thousands of years.

Quick check

Question 1 of 3

What does a wave carry?

Words to know

Wave: A pattern that carries energy from place to place.

Wave: A pattern that carries energy from place to place.
Wavelength: The distance from one wave crest to the next.
Frequency: How many waves pass each second. It is measured in hertz.

Meet the scientist

Zhang Heng

Next up

Sound: vibrations you can hear