Have you ever watched ripples spreading out when you drop a pebble into a pond? We can recreate and measure these waves in a lab using a ripple tank filled with water to a depth of 5 mm to 1 cm. A motor-driven vibrating bar creates plane waves across the surface.
A lamp placed above the tank projects shadows of the ripples onto white card or paper below. The bright spots on the paper correspond to the wave crests, which act as convex lenses to focus the light, forming a visible wavefront.
First, to find the wavelength, measure the distance across a large number of wavefronts (e.g., 10 waves) using a metre ruler on the paper, then divide by the number of waves. Next, to find the frequency, count the number of waves passing a fixed point in a set time (e.g., 10 seconds) and divide by the time taken. Finally, calculate the wave speed using the wave equation:
To improve this method, you can use a stroboscope matched to the frequency of the motor. This makes the waves appear "frozen", which removes the difficulty of measuring a moving target and reduces parallax error.
You can snap a piece of chalk, but try snapping a diamond. Solids transmit waves differently depending on their properties. To measure waves in a solid string, you need a signal generator, a vibration generator, a string over a wooden bridge, a pulley, and a mass hanger.
When the signal generator is turned on, waves travel down the string and reflect back, superposing to form a standing wave. This pattern features nodes, which are points with zero displacement, and antinodes, which are points with maximum displacement.
Read the frequency directly from the signal generator. Measure the total length of multiple visible loops using a ruler; one loop (the distance between two adjacent nodes) represents exactly half a wavelength (). Calculate the full wavelength, then multiply by the frequency to find the speed.
Worked Example
Calculations require precision. A student measures the total length of 3 loops as . The frequency is . Calculate the wave speed.
Step 1: Calculate the length of one loop.
Step 2: Calculate the full wavelength ().
Step 3: Substitute into the wave equation ().
Lightning and thunder happen at exactly the same time, but why do you always see the flash before you hear the boom? Sound travels fast, but we can measure its exact speed using high-frequency sound waves (e.g., ) and two microphones connected to a dual-trace oscilloscope.
First, place both microphones in front of a loudspeaker so their traces align on the screen, meaning they are in phase. Then, move one microphone away from the speaker until the two traces align exactly once more. The distance between the microphones is exactly one wavelength.
Alternatively, you can place the microphones at a known distance and use the oscilloscope's timebase to measure the time delay for a sound pulse to travel between them. The speed is then calculated by dividing the distance by the time delay.
Shouting into a canyon and hearing your voice bounce back is a classic example of an echo. To use echoes to measure the speed of sound, stand at least 50 metres from a tall, vertical wall. Measure this distance using a trundle wheel, as it is far more practical than a standard metre ruler over long distances.
A person claps two wooden blocks together in rhythm with the echoes they hear returning from the wall. A second person uses a stopwatch to time a large number of claps (e.g., 50 or 100 claps) to calculate the total time taken.
Worked Example
A student stands from a wall. They time 20 claps. The total time taken is . Calculate the speed of sound.
Step 1: Calculate the total distance. Remember sound travels to the wall AND back for every clap.
Step 2: Use the speed formula ().
Would you use a plastic ruler to measure the thickness of a human hair? Choosing the right tool is vital in physics to ensure reliable results and low uncertainty.
A standard stopwatch has a resolution of , but human reaction time is typically to . This introduces significant random error for fast events like sound traveling short distances. An oscilloscope is highly suitable here because its microsecond resolution entirely eliminates human reaction time.
For ripple tanks, recording a slow-motion video with a timer in the frame allows for frame-by-frame analysis. This massively reduces systematic observation errors compared to manual counting. In all methods, measuring across multiple waves (10 wavefronts or 10 loops) is the best way to reduce the percentage uncertainty of the final result.
When calculating the speed of sound using an echo, students frequently forget to double the distance. Always remember that the sound must travel to the wall and back again for an echo to be heard.
In 'Evaluate' questions comparing apparatus, examiners expect you to state BOTH the advantage of the superior equipment (e.g., the oscilloscope has a higher resolution) AND why the alternative is flawed (e.g., a stopwatch is limited by human reaction time).
Whenever you are describing a wave experiment, explicitly mention measuring across multiple waves (e.g., 10 wavefronts, 10 loops, or 50 claps) and dividing by that number. This is the standard method to reduce percentage uncertainty and secure high marks.
Wavefront
A line representing all the points on a wave that are in the same phase, such as a line connecting all the wave crests.
Wavelength
The distance from a point on one wave to the equivalent point on the adjacent wave, measured in metres (m).
Frequency
The number of waves passing a fixed point per second, measured in Hertz (Hz).
Parallax error
A systematic error caused by viewing a measurement scale at an angle rather than straight on.
Standing wave
A stationary wave pattern formed when two waves of the same frequency travelling in opposite directions superpose.
Node
A point on a standing wave that has zero displacement (no vibration).
Antinode
A point on a standing wave that has maximum displacement.
Oscilloscope
A device that displays a voltage-time graph of an electrical signal, used to accurately measure high-frequency waves.
In phase
When the peaks and troughs of two waves occur at exactly the same time and position.
Timebase
The control on an oscilloscope that determines the time represented by the horizontal divisions on the screen.
Uncertainty
The interval within which the true value of a measurement is expected to lie.
Resolution
The smallest change in a quantity that gives a perceptible change in the reading on a measuring instrument.
Random error
An error that causes readings to be spread about the true value, due to results varying in an unpredictable way from one measurement to the next.
Put your knowledge into practice — try past paper questions for Physics
Wavefront
A line representing all the points on a wave that are in the same phase, such as a line connecting all the wave crests.
Wavelength
The distance from a point on one wave to the equivalent point on the adjacent wave, measured in metres (m).
Frequency
The number of waves passing a fixed point per second, measured in Hertz (Hz).
Parallax error
A systematic error caused by viewing a measurement scale at an angle rather than straight on.
Standing wave
A stationary wave pattern formed when two waves of the same frequency travelling in opposite directions superpose.
Node
A point on a standing wave that has zero displacement (no vibration).
Antinode
A point on a standing wave that has maximum displacement.
Oscilloscope
A device that displays a voltage-time graph of an electrical signal, used to accurately measure high-frequency waves.
In phase
When the peaks and troughs of two waves occur at exactly the same time and position.
Timebase
The control on an oscilloscope that determines the time represented by the horizontal divisions on the screen.
Uncertainty
The interval within which the true value of a measurement is expected to lie.
Resolution
The smallest change in a quantity that gives a perceptible change in the reading on a measuring instrument.
Random error
An error that causes readings to be spread about the true value, due to results varying in an unpredictable way from one measurement to the next.