Waves

A wave is a repeating disturbance that carries energy from one place to another without moving matter along with it.

Reading is good — doing is better. Practice Waves as an interactive lesson.

Definition

A wave is a back-and-forth or up-and-down disturbance that travels through a material or through space, transferring energy from one spot to another. The material the wave moves through (like water or air) does not travel with the wave — only the energy does.

Remember the rule

More amplitude = more energy. Higher frequency = more waves per second.

Key words

Wave: A repeating disturbance that moves energy from place to place.
Crest: The highest point of a wave.
Trough: The lowest point of a wave.
Wavelength: The distance from one crest to the next crest (or one trough to the next trough).
Amplitude: The height of a wave from its resting position to its crest — bigger amplitude means more energy.
Frequency: How many waves pass a point in one second — more waves per second means higher frequency.
Medium: The material a wave travels through, like water, air, or a rope.
Vibration: A quick back-and-forth movement that creates a wave.

Worked examples

You drop a pebble into a still pond. What happens, and does the water travel outward?

→ Ripples (water waves) spread out in circles from where the pebble hit. The water itself moves up and down but does NOT travel outward — only the energy of the wave moves across the pond. · This is the key proof that waves carry energy, not matter.

Your friend shakes one end of a long rope up and down once. What travels to the other end?

→ A single wave bump (called a pulse) travels along the rope to your end. The rope itself stays in place — each part just moves up and then down as the wave passes. · The rope is the medium; the energy is what moves.

A wave has a crest 30 cm above the resting line and a trough 30 cm below it. What is the amplitude?

→ The amplitude is 30 cm — measured from the resting (middle) position up to the crest. · Amplitude is NOT the full height from trough to crest (that would be 60 cm); it is only half of that.

You see 4 full waves pass a dock post in 1 second. What is the frequency?

→ The frequency is 4 waves per second (written as 4 Hz). Each complete wave — one crest and one trough — counts as one full wave.

Two ocean waves are shown. Wave A has a tall crest; Wave B has a short crest. Which wave carries more energy?

→ Wave A carries more energy because it has a larger amplitude. Bigger amplitude always means more energy in a wave.

A sound wave travels from a speaker through the air to your ear. What is the medium?

→ The medium is air. The sound wave is a vibration that pushes air particles back and forth, carrying the energy of the sound from the speaker to your ear.

Common mistakes

Thinking the water or rope moves forward with the wave — the medium only moves up and down (or back and forth), not in the direction the wave travels.
Confusing amplitude with wavelength — amplitude is the height of a wave; wavelength is the distance between two crests.
Thinking a quieter sound means lower frequency — volume (loudness) is about amplitude, not frequency. Frequency controls pitch (high or low sound).
Measuring amplitude from the trough all the way to the crest — amplitude is only from the rest position to the crest, which is half that full distance.
Thinking waves need a medium to travel — light waves (and other electromagnetic waves) can travel through empty space with no medium at all.

FAQs

Do all waves need something to travel through?

No. Sound waves need a medium like air or water, but light waves can travel through the vacuum of outer space with nothing to travel through at all.

What makes a wave louder or brighter?

A bigger amplitude. In a sound wave, bigger amplitude means louder sound. In a light wave, bigger amplitude means brighter light.

What is the difference between frequency and wavelength?

Frequency is how many waves pass a point each second. Wavelength is the physical length of one complete wave. When frequency goes up, wavelength gets shorter, and vice versa.

Why do ocean waves knock you down but the water does not carry you straight to shore?

The water under you moves in a circle — up, forward, down, and back — as each wave passes. The energy moves toward shore, but the water returns to nearly where it started, which is why you bob but do not get pushed steadily in one direction.

What starts a wave?

A vibration. Something has to move back and forth — like a speaker cone, your vocal cords, or a pebble hitting water — to create the disturbance that becomes a wave.

Is sound a wave?

Yes! Sound is a wave made of vibrating air particles. When you speak, your vocal cords vibrate and push air particles together and apart in a pattern that travels to a listener's ear as a sound wave.

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