Waves and Diffraction
A wave is a disturbance that transfers energy from one place to another.
Examples of waves include sound waves, water waves and electromagnetic radiation
There are 2 types of waves described in national 5:
Transverse Waves
Transverse waves are the “wavey” looking waves and occur when particles vibrate at a right angle to the direction of the wave. Examples include water waves and electromagnetic radiation.
Longitudinal Waves
Longitudinal waves occur when particles vibrate in the same direction of the wave. This can be shown diagrammatically as straight lines and an example of longitudinal waves are sound waves.
Wave Definitions
Frequency - The number of waves produced per second.
Wavelength - The length from one point of one wave to the same point on the next wave. This may be from the start of a wave to the start of the next wave, or from the peak of one wave to the peak of the next wave, trough to trough, etc.
Amplitude - Half of the distance from the peak of the wave to the trough.
Period - Time taken for a complete wave to be produced or one whole wave to pass a point.
Wave Speed - How fast a wave travels from its source in a given amount of time
Period and frequency equation
The first wave equation involves the relationship between Frequency and Period.
So when given either frequency and period, consider that this could be an equation to use in a question.
Eg. A wave has a period of 5s, calculate its frequency.
Wave Speed Calculation
Wave speed can be calculated simply using the formula v = d/t.
The speed of sound waves in air is 340m/s
The speed of light in a vacuum is 300,000,000m/s (3x10^8m/s) - this is a constant and doesn’t change.
Knowing the speed values of sound and light allows you to calculate the distance from the wave sources. So if a firework was to go off and you heard it 2 seconds later, the distance would be the speed of sound x time = 340 x 2 = 680m.
Wave Equation
This equation helps describe waves and will almost definitely come up in your exams. It describes the relationship between wave speed, frequency and wavelength.
Wave speed = frequency x wavelength.
REMEMBER - speed of light is 3x10^8m/s in a vacuum (and air) and speed of sound is 340m/s in air.
Practice changing the subject of the formula between frequency, wavelength and wave speed, as a variation of the wave equation is almost a guaranteed question in exams.
Diffraction
Waves have the ability to bend around objects that they encounter. This is how radio waves can travel around mountains and buildings, without being blocked, to reach your satellite dish to let you watch Love Island. This ability is called Diffraction.
It is important to define diffraction and to be able to draw it diagrammatically.
Diffraction, more specifically occurs when waves encounter and object or a gap.
Longer wavelengths (smaller frequencies) are able to bend more than shorter wavelengths (higher frequencies).
When a wave diffracts, the frequency and wavelength stay the same.
Key Points!
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Types of Waves
Transverse waves occur when particles vibrate at a right angle to the direction of the wave. Eg water waves, electromagnetic radiation (light).
Longitudinal waves occur when particles vibrate in the same direction of the wave. Eg sound waves.
You need to be able to draw both transverse and longitudinal waves.
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Wave Definitions
Frequency - The number of waves produced per second.
Wavelength - The length from one point of one wave to the same point on the next wave.
Amplitude - Half of the distance from the peak of the wave to the trough.
Period - Time taken for a complete wave to be produced or one whole wave to pass a point.
Wave Speed - How fast a wave travels from its source in a given amount of time
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Period and Frequency
Frequency = 1 / Period
Be able to rearrange this equation and remember the units.
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Wave Speed
Speed of light in vacuum - 3x10^8m/s
Speed of sound in air - 340m/s
Wave speed = distance / time
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Wave Equation
Wave speed = frequency x wavelength
Be able to rearrange this equation.
Understand that wave speed of light in a vacuum and sound in air are constants, meaning that a change in frequency means a change in wavelength. Frequency and wavelength are inversely proportional.
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Diffraction
When waves encounter an object or a gap, they can bend around it.
Waves with shorter wavelengths bend around objects (diffract) less than waves with longer wavelengths.
Be able to draw diffraction diagrammatically.
Diffraction doesn’t cause a change in frequency or wavelength.