Properties of Waves

Wave goodbye to confusion about the different types of waves, this page has got you covered. Here you can read about the difference between longitudinal and transverse waves, their properties and how they can be reflected and refracted.

Longitudinal and transverse waves

Waves transfer information and energy from one place to another without transferring matter. For example, light waves will transfer light energy from its source (a lamp) outwards. Waves move through vibrations, which can move in one of two ways. The vibrations can move either up and down (a transverse wave) or side to side (a longitudinal wave).

Transverse waves include light waves and other types of electromagnetic waves. The vibrations are at right angles (perpendicular) to the direction of wave travel. If you imagine taking a slinky and shaking it up and down, or simply the movement of waves at sea, then you’re picturing a transverse wave.

Longitudinal waves include sound and ultrasound waves. The vibrations are in the same direction (parallel) to the direction of wave travel. Longitudinal waves can be represented by pushing a spring back and forth. We call the areas where the particles are bunched up ‘compressions’ and the areas where the particles are more spread out ‘rarefactions’.

Parts of a of wave

The following terms are used to describe the different parts of a wave and you need to know what they all mean:

Peak - the highest point of a wave.
Trough - the lowest point of a wave.
Wavelength - the length of one wave i.e. the distance from peak to peak (or trough to trough).
Frequency - the number of waves passing a point each second, measured in hertz (Hz). 1 Hz is equivalent to 1 wave per second. Frequency is related to pitch, the larger the frequency the higher the pitch.
Amplitude - the height of a wave, measured from the resting point to its peak. Amplitude is related to volume, the larger the amplitude, the higher the volume.
Period - the time taken in seconds for one complete wave to pass a point

Calculating frequency

Frequency is a measure of the number of waves passing a point each second. It can be calculated using the following equation:

Worked example: calculating wave frequency

Calculate the frequency of a wave which has a time period of 0.04 seconds.

Frequency = 1 / time (in seconds)
Frequency = 1 / 0.04 = 25 Hz
This means that 25 ‘wavelengths’ passed a point in one second

You may get a question where you are given the frequency and asked to calculate the time period. In this case you simply rearrange the equation to give: Time = 1 / frequency.

Speed, frequency and wavelength

The speed of a wave can be calculated using this equation:

Worked example: calculating wave speed

What is the speed of a wave which has a wavelength of 8 meters and a frequency of 50 Hz?

Speed = frequency x wavelength
Speed = 50 x 8 = 400 m/s

Sound waves

Sound waves are longitudinal waves, which means they travel through vibrations moving back and forth (i.e. the vibrations are parallel to the direction of wave travel). The speed of sound depends on the medium through which it is travelling, for example, sound waves travel over four times faster in water compared to air. Sound waves require particles to carry the vibrations, which means they cannot travel through a vacuum (as there are no particles).

Electromagnetic waves

Electromagnetic (EM) waves are transverse waves, which means the vibrations move up and down (at right angles) to the direction of wave travel. Unlike sound waves, they do not require particles to carry the vibrations which means they are able to travel through a vacuum. There are seven different types of EM waves which form are organised along a spectrum - we call this the electromagnetic spectrum. All EM waves travel the same speed through a vacuum but they have different wavelengths and frequencies. You’ll learn more about EM waves on the next page.

Wavefronts

When a source emits a wave, it’s not just an individual wave which is emitted but several that are travelling in the same direction. A ‘wavefront’ is an imaginary line that is drawn across the peaks of the different waves. The next wavefront would be drawn at the next peak. The distance between each wavefront is equal to one wavelength. In the diagram illustrating the Doppler Effect below, the wavefronts are represented as black circles.

The Doppler Effect

The Doppler Effect occurs when a wave is emitted from a moving source. It appears as if the frequency of the wave changes as the source moves past you. You will have experienced the Doppler Effect when an ambulance or police car with its siren blaring passes you - the pitch of the siren sounds higher when it comes towards us and lower when it is travelling away from us.

This happens because the sound waves have a constant wave speed. As the ambulance moves, it approaches the sound waves ahead of it, causing the wavefronts to become closer together (they now have a shorter wavelength) while the wavefronts behind the vehicle become more spread out (they now have a longer wavelength). The change in wavelengths result in a change in frequency. Remember the equation: speed = frequency x wavelength, so if the wavelength decreases but speed stays the same, then the frequency must increase.

Reflection and refraction

All waves can be reflected and refracted. Reflection occurs when waves bounce off a surface whereas refraction is when wave speed changes when a wave travels through a different material. Read more about reflection and refraction here.