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P6 A) Wave Introduction
P6 A) Wave Introduction
Waves transfer energy from one place to another. Waves do not transfer any matter; they only transfer energy.
Some waves must travel through a medium. For example, sound needs to travel through a medium, which on earth is air; space is silent because there is no air in space, so no medium for the sound waves to travel through (space is a vacuum). Other waves do not need to travel through a medium and can travel through a vacuum. For example, electromagnetic waves from the sun can travel through a vacuum and reach us on earth; parts of the electromagnetic spectrum that reach earth from the sun are visible light, ultraviolet light, infrared, radio waves, x-rays and gamma rays.
When a wave travels through a medium (solid, liquid or gas), the particles of the medium oscillate and transfer energy, but the particles stay in the same position and do not travel with the wave. For example, a ball being dropped in water causes ripples to form.
Some waves must travel through a medium. For example, sound needs to travel through a medium, which on earth is air; space is silent because there is no air in space, so no medium for the sound waves to travel through (space is a vacuum). Other waves do not need to travel through a medium and can travel through a vacuum. For example, electromagnetic waves from the sun can travel through a vacuum and reach us on earth; parts of the electromagnetic spectrum that reach earth from the sun are visible light, ultraviolet light, infrared, radio waves, x-rays and gamma rays.
When a wave travels through a medium (solid, liquid or gas), the particles of the medium oscillate and transfer energy, but the particles stay in the same position and do not travel with the wave. For example, a ball being dropped in water causes ripples to form.
The ripples that form from the ball being dropped do not transfer water or energy away with them; instead, the water just transfers energy.
Parts of a Wave
We are now going to have a look at the different parts of a typical wave.
We are now going to have a look at the different parts of a typical wave.
Here are some key terms for waves:
- Rest position – this is the undisturbed position of the particles or field when they are not vibrating/ oscillating. The rest position is the x-axis.
- Displacement – the displacement is worked out with respect to the rest position. It is the distance that a point in the medium has moved from its rest position. The displacement is measured on the y-axis.
- Crest or peak – the highest point above the rest position.
- Trough – the lowest point below the rest position.
- Amplitude – the maximum displacement between the rest point (the x-axis) and either the crest (peak) or the trough.
- Wavelength – the length of a full cycle of the wave. It is easier to measure the wavelength using a trough or a crest as during one full wavelength the wave only crosses it once. If you use a rest point on the wave, one full wavelength will cross the rest point twice; this will make more sense when we look at example 1.
- Frequency – the number of complete waves that pass through a certain point each second. Frequency is measured in hertz (Hz), and 1 Hz is one full wave per second. If a wave has a very high frequency, we may measure its frequency in kilohertz (1 KHz = 1 thousand Hz), megahertz (1 MHz = 1 million Hz) or gigahertz (1 GHz = 1 billion Hz).
Example 1
The graph below is for a wave in water.
The graph below is for a wave in water.
Answer the following:
a) What is the amplitude?
b) What is the wavelength?
Part a
The amplitude is the maximum displacement between the rest point (the x-axis) and either the crest (peak) or the trough. It does not matter whether we measure the distance between the rest point and the peak, or the rest position and the trough. The amplitude for the wave above is 1.5 cm. I have drawn the amplitude on for both the peak and the trough.
a) What is the amplitude?
b) What is the wavelength?
Part a
The amplitude is the maximum displacement between the rest point (the x-axis) and either the crest (peak) or the trough. It does not matter whether we measure the distance between the rest point and the peak, or the rest position and the trough. The amplitude for the wave above is 1.5 cm. I have drawn the amplitude on for both the peak and the trough.
Part b
The wavelength is the length of a full cycle of the wave. I am going to measure the wavelength from the crest to the crest.
The wavelength is the length of a full cycle of the wave. I am going to measure the wavelength from the crest to the crest.
From the above working, we can see that the wavelength is 4 cm.
We could have also measured the wavelength from the trough or from the rest point. If you are using the rest point on the wave, you need to calculate the wave crossing the rest point twice; the wave will need to go up to the crest and back to the rest point, and then down to the trough and back up to the rest point (or in the opposite order; rest point, trough, rest point, crest, rest point). The wavelength when measured from the rest point is shown below.
We could have also measured the wavelength from the trough or from the rest point. If you are using the rest point on the wave, you need to calculate the wave crossing the rest point twice; the wave will need to go up to the crest and back to the rest point, and then down to the trough and back up to the rest point (or in the opposite order; rest point, trough, rest point, crest, rest point). The wavelength when measured from the rest point is shown below.
From the above working, you can see that the wavelength is 4 cm, which is the same as it was when we worked out the wavelength from crest to crest.
Transverse & Longitudinal Waves
There are two types of waves; transverse waves and longitudinal waves. We are going to use a slinky toy to help us understand the difference between transverse and longitudinal waves.
Transverse Waves
Transverse waves have oscillations that are perpendicular (right angles) to the direction of wave travel. So, if a transverse wave was travelling towards the right, the oscillations would be up and down (perpendicular to the direction of travel).
There are two types of waves; transverse waves and longitudinal waves. We are going to use a slinky toy to help us understand the difference between transverse and longitudinal waves.
Transverse Waves
Transverse waves have oscillations that are perpendicular (right angles) to the direction of wave travel. So, if a transverse wave was travelling towards the right, the oscillations would be up and down (perpendicular to the direction of travel).
We are now going to use the slinky to show a transverse wave. If we held the slinky with both hands and then lifted one of our hands up, we would see a wave being created in the slinky.
Here are some examples of transverse waves:
- All electromagnetic waves, such as visible light, radio waves, x-rays, gamma rays etc…
- Ripples on the surface of water
- A wave in a string, which can be a rope or a musical string (guitar or violin)
Longitudinal Waves
Longitudinal waves have oscillations that are parallel to the direction of wave travel. So, if a transverse wave was travelling towards the right, the oscillations would be left and right (parallel to the direction of travel).
Longitudinal waves have oscillations that are parallel to the direction of wave travel. So, if a transverse wave was travelling towards the right, the oscillations would be left and right (parallel to the direction of travel).
We are now going to use the slinky to show a longitudinal wave. We start by holding our slinky with both hands, and then we push one of our hands against the end of the slinky. This will result in the springs in the slinky becoming closer in some areas and further away in other areas.
All longitudinal waves have areas of compression and rarefaction.
- Compressions are regions of high pressure due to the particles being close together. For the slinky, this is where the springs are very close to each other.
- Rarefactions are regions of low pressure due to the particles being further away from each other. For the slinky, this is where the springs are far away from each other.
Here are some examples of longitudinal waves:
- Sound waves
- Ultrasound