Wave Properties Lab

What a wave carries

Drop a stone into a pond and rings spread outwards, but a floating leaf only bobs up and down. It does not travel to the edge with the ripples. That is the whole idea of a wave: it carries energy from one place to another WITHOUT carrying the material along with it. Sound reaching your ear, light from a lamp and ripples on water are all waves. To describe any of them you need the same handful of measurements, and one equation ties them together. This module covers what those measurements mean, the two families that waves come in, and how to find the speed of a wave, including in a ripple tank in the lab.

Words for waves

Learn what each of these measures before you use it.

Read the frequency

A wave on a stretched spring is shaken so that four complete waves pass a fixed point every second. What is its frequency?

• 4 hertz
• 0.25 hertz
• 4 metres
• 4 seconds

Two families of wave

Every wave is one of two kinds, and the difference is the direction the material vibrates compared with the direction the wave travels.

Match the wave word to its meaning

Spot the true wave facts

Tap the TWO statements that are true.

• Wave speed is found by multiplying the frequency by the wavelength
• Sound is a longitudinal wave
• Light is a longitudinal wave
• The amplitude is the distance between two crests

Order the ripple-tank method

Put the steps for measuring the speed of water waves in a ripple tank in order.

• Make waves at a steady rate and count how many pass a point each second to get the frequency
• Measure the length of one wave to get the wavelength
• Multiply the frequency by the wavelength
• Read off the wave speed in metres per second

Work out the wave speed

A wave has a frequency of 5 hertz and a wavelength of 2 metres. Wave speed equals the frequency multiplied by the wavelength. Multiply 5 by 2 to find the wave speed in metres per second.

How to measure any wave

You can find any wave measurement by watching one point and one length. To get the frequency, count how many complete waves pass a single point in one second. To get the wavelength, measure the length of exactly one wave, from any point to the matching point on the next one. Once you have those two, the speed follows from the equation, and the units check themselves: waves each second multiplied by metres gives metres each second. If a question gives you the speed and one other value instead, rearrange the same equation by dividing. Measure carefully first, then reach for the numbers.

Complete the wave facts

The _____ of a wave is its maximum displacement from the rest position. The number of complete waves passing a point each second is the _____. Wave speed is found by multiplying the frequency by the _____. In a _____ wave the vibrations are at right angles to the direction of travel.

Build the wave-speed rule

Choose the words that complete the rule for finding the speed of a wave.

Plot speed against frequency

A wave has a fixed wavelength of 2 metres, so its speed is always the frequency multiplied by 2. Plot the wave speed for each frequency: at frequency 1 plot 2, at frequency 2 plot 4, at frequency 3 plot 6, at frequency 4 plot 8, and at frequency 5 plot 10.

A ripple-tank calculation

Here is a calculation worked through so the method is clear. In a ripple tank, four complete waves pass a point every second, so the frequency is 4 hertz, and each wave is measured to be 3 metres long, so the wavelength is 3 metres. What is the wave speed. Wave speed equals the frequency multiplied by the wavelength, so that is 4 multiplied by 3, which gives 12 metres per second. Now suppose you slow the waves down until the wavelength doubles to 6 metres while the speed stays 12 metres per second. To find the new frequency, rearrange the equation and divide the speed by the wavelength: 12 divided by 6 gives a frequency of 2 hertz. Notice the check: longer waves at the same speed must arrive less often, and a frequency of 2 hertz is indeed less than the 4 hertz you started with.

Describe the waves

Read each case and choose the best answer, then think about why.

• A sound wave travels through the air. In which direction do the air particles vibrate?
• In a ripple tank, three complete waves pass a point each second and each wave is 2 metres long. What is the wave speed?
• You want to measure the speed of the water waves in a ripple tank. What is the best method?

Waves checkpoint

Select the TWO statements that are true.

• Wave speed equals frequency multiplied by wavelength
• In a transverse wave the vibrations are at right angles to the direction of travel
• Longitudinal waves have crests and troughs
• The amplitude is the distance between two crests

Explain waves

A friend is revising Physics and is muddled about waves. Explain how waves are described and measured, using the ideas from this module.

• Explain what amplitude, wavelength and frequency each measure
• Explain the difference between a transverse wave and a longitudinal wave, with an example of each
• State the wave-speed equation in words
• Give a worked example of calculating a wave speed from a frequency and a wavelength
• Explain how you could measure the speed of water waves in a ripple tank