What is moving in a wave?
Watch a stadium crowd do “the wave”. A ripple races around the stadium — but nobody leaves their seat. Each person just stands up and sits down. What travels is a pattern, not the people.
A wave carries energy from place to place, without carrying the stuff it moves through.
The same is true for ripples on a pond, a wiggle sent down a rope, sound through air, and (with a twist) light. Let's see it happen.
A string being shaken at its left end, sending waves to the right where the end is fixed or loose. Choose a continuous shake or a single pulse and set the frequency, amplitude and wave speed. Readings show the wave speed, frequency and wavelength.
Wave on a string
InteractiveDriver
Right end
The math, with your numbers
Shake faster (higher f) and the waves get shorter — the speed stays the same.
Try this
- →Watch the green bead. Does it travel along the string, or just bob up and down?
- →Double the frequency. What happens to the wavelength? To the speed?
- →Choose 'Single pulse' and 'Tied to a wall'. What happens when the pulse hits the wall?
The parts of a wave
- Amplitude — how big the wiggle is (the height of a crest above the middle). Bigger amplitude = more energy.
- Wavelength (lambda) — the distance from one crest to the next.
- Frequency — how many waves pass a point each second, measured in hertzOne cycle per second. Named after Heinrich Hertz, who first produced radio waves in a lab (1887). (Hz).
- Wave speed — how fast the pattern moves. For a string, it depends on how tight and how heavy the string is — not on how you shake it.
The wave equation
Say it out loud: “wave speed equals frequency times wavelength.”
Wave speed· measured in m/s
How fast the pattern travels.
Frequency· measured in Hz = 1/s
Waves produced per second.
Wavelength· measured in m
Length of one wave.
Make a prediction first
On the same string (same wave speed), you shake twice as fast. What happens to the wavelength?
Think about it, then click to reveal the answer.
Make a prediction first
On the same string (same wave speed), you shake twice as fast. What happens to the wavelength?
Think about it, then click to reveal the answer.
It halves. The speed is fixed by the string, so if doubles, must halve. Try it in the simulation: crank up the frequency and the waves bunch up. This is exactly why high-pitched sounds have short wavelengths and low-pitched sounds have long ones.
Worked example: the musical note A (440 Hz) in air
Step 1 of 3- 1Sound travels through room-temperature air at about 343 m/s.
Reflections and the wider world
Switch to Single pulse and Tied to a wall. The pulse bounces back upside-down! The wall can't move, so when the pulse pulls up on it, the wall pulls down on the string (Newton's third law again), launching an inverted pulse back. Echoes are reflections of sound waves.
Wrap-up
Key ideas to remember
- A wave carries energy, not matter. Each part of the medium only moves back and forth.
- Waves have amplitude, wavelength λ, frequency f, and speed v.
- : in one period, the wave travels one wavelength.
- The medium sets the speed; the source sets the frequency; together they set the wavelength.
Check your understanding
Sources & further reading
Everything in this lesson agrees with these references. They're all free to read.