Toss a pebble into a still pond and watch the circles spread outwards. The ripples that travel across the surface of the water are a perfect model of a transverse wave. In a transverse wave, the oscillation of the particles is perpendicular (at right angles) to the direction of energy transfer.
First, a disturbance hits the water surface. Then, the water particles begin to vibrate vertically, moving up and down. Finally, this up-and-down motion pushes the wave horizontally outward from the source. You can prove that it is the wave energy travelling and not the water itself by placing a small floating object, like a cork, on the surface. The cork will simply bob up and down as the ripples pass beneath it, but it will not travel forward with the wave.
When a speaker blasts heavy bass at a concert, you can actually feel the air vibrating against your chest. This occurs because sound waves are a longitudinal wave, meaning the oscillations of the particles are parallel to the direction of energy transfer. Sound is a mechanical wave, so it strictly requires a physical medium (like a solid, liquid, or gas) to travel through and cannot propagate in a vacuum.
First, a sound source vibrates, pushing into the surrounding air. Then, the air particles oscillate back and forth in the exact same direction that the wave is travelling. This parallel motion creates alternating regions in the air: compression, where particles are squashed closely together to create high pressure, and rarefaction, where particles are spread further apart, creating low pressure. A slinky spring is often used to model this, where a pushed coil creates a visible pulse moving along the spring. Importantly, sound waves do not create a flow of air or "wind" moving from the source to your ear; the particles simply vibrate around a fixed point.
How do we measure the speed of something that is constantly moving and changing shape? In a laboratory, a ripple tank is used to measure the properties of water waves. A shallow glass-bottomed tank is filled with water, and a wooden bar connected to a vibrating motor creates straight ripples. A lamp positioned above the tank shines light through the water, and the wave crests act like lenses, projecting bright shadows onto a white screen below.
To find the wave speed using the formula , you must measure both the Wavelength (λ) and the Frequency (f):
A student uses a ripple tank to investigate water waves. They measure the distance across 8 complete waves to be . They count 12 waves passing a fixed mark in . Calculate the speed of the water waves.
Step 1: Calculate the wavelength ().
Step 2: Calculate the frequency ().
Step 3: Use the wave equation to calculate speed ().
Shouting into a vast canyon and waiting for your voice to bounce back is more than just fun—it is the basis for a classic physics experiment. The speed of sound in air can be measured by calculating how long it takes for an echo to travel over a known distance.
First, stand a long distance away (typically or ) from a tall, flat wall. Measure this distance precisely using a trundle wheel. Next, one person strikes two wooden blocks together to create a sharp, loud sound. A second person stands alongside them holding a stopwatch. Because light travels much faster than sound, the timer starts the stopwatch the exact moment they see the blocks hit, and stops the stopwatch the moment they hear the echo return from the wall. The speed is then calculated using distance divided by time. To reduce errors caused by human reaction time, the person clapping can synchronize their claps with the returning echoes over a set of 20 intervals, and the total time can be measured.
Two students stand from a large brick building. One student hits two blocks together, and the other measures a time of between seeing the impact and hearing the echo. Calculate the speed of sound in air.
Step 1: Calculate the total distance the sound wave travelled.
Step 2: State the formula for speed.
Step 3: Substitute the values and calculate.
Students often forget to double the distance in the echo experiment. The sound wave has to travel to the wall AND bounce back to your ears, so multiply the distance to the wall by two before calculating speed.
In 6-mark questions comparing wave types, OCR strictly requires the exact word 'perpendicular' to describe transverse waves and 'parallel' to describe longitudinal waves.
Whenever you are asked how to improve the accuracy of measuring wavelength in a ripple tank, always state that you would measure across multiple waves (e.g., 10 waves) and divide by that number to find the length of one wave.
Examiners frequently ask for evidence that waves transfer energy and not matter; explicitly state that a floating object on water bobs up and down but does not travel forward with the wave.
Transverse wave
A wave in which the oscillations of the particles are perpendicular (at right angles) to the direction of energy transfer.
Longitudinal wave
A wave in which the oscillations of the particles are parallel to the direction of energy transfer.
Oscillation
A repetitive back-and-forth or up-and-down vibrating motion about a central equilibrium position.
Perpendicular
At right angles (90°) to a given line or direction.
Parallel
Moving in the same direction or on the same line as another direction.
Mechanical wave
A wave that requires a physical medium (particles) to travel through and cannot propagate through a vacuum.
Compression
A region in a longitudinal wave where the particles are closest together, creating an area of high pressure.
Rarefaction
A region in a longitudinal wave where the particles are spread further apart, creating an area of low pressure.
Wavelength (λ)
The distance from one point on a wave to the exact equivalent point on the next adjacent wave.
Frequency (f)
The number of complete waves passing a fixed point per second, measured in Hertz (Hz).
Stroboscope
A device that emits regular flashes of light, used to make rapidly moving objects (like fast water waves) appear stationary.
Echo
A sound wave that has been reflected off a solid surface and returns to the listener.
Trundle wheel
A measuring device consisting of a wheel with a known circumference attached to a handle, used to measure long distances along the ground.
Reaction time
The time it takes for a human to respond to a visual or auditory stimulus, which can introduce errors in manual timing experiments.
Put your knowledge into practice — try past paper questions for Physics A
Transverse wave
A wave in which the oscillations of the particles are perpendicular (at right angles) to the direction of energy transfer.
Longitudinal wave
A wave in which the oscillations of the particles are parallel to the direction of energy transfer.
Oscillation
A repetitive back-and-forth or up-and-down vibrating motion about a central equilibrium position.
Perpendicular
At right angles (90°) to a given line or direction.
Parallel
Moving in the same direction or on the same line as another direction.
Mechanical wave
A wave that requires a physical medium (particles) to travel through and cannot propagate through a vacuum.
Compression
A region in a longitudinal wave where the particles are closest together, creating an area of high pressure.
Rarefaction
A region in a longitudinal wave where the particles are spread further apart, creating an area of low pressure.
Wavelength (λ)
The distance from one point on a wave to the exact equivalent point on the next adjacent wave.
Frequency (f)
The number of complete waves passing a fixed point per second, measured in Hertz (Hz).
Stroboscope
A device that emits regular flashes of light, used to make rapidly moving objects (like fast water waves) appear stationary.
Echo
A sound wave that has been reflected off a solid surface and returns to the listener.
Trundle wheel
A measuring device consisting of a wheel with a known circumference attached to a handle, used to measure long distances along the ground.
Reaction time
The time it takes for a human to respond to a visual or auditory stimulus, which can introduce errors in manual timing experiments.