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AQA A-Level Physics Topic 3 - Waves; made directly in accordance with the AQA 7408 specification
This is a draft cheat sheet. It is a work in progress and is not finished yet.
Waves
A wave is a means of transferring energy and momentum from one point to another without there being any transfer of matter between the two points. |
They can be mechanical/electromagnetic, progressive/stationary and longitudinal/transverse |
Types of waves
Mechanical waves are made up of vibrating particles. They require a substance for transmission, so can't travel through a vacuum.
eg sound (air molecules), water (water molecules) |
Electromagnetic waves are made up of oscillating electric and magnetic fields. They don't require a substance for transmission, so can travel through a vacuum.
eg light and radio |
Progressive waves are when there is a net transfer of energy and momentum from one point to another
eg sound from a person speaking, light from a lamp |
Stationary waves are when there is NO net transfer of energy and momentum from one point to another.
eg the wave on a guitar string |
Longitudinal waves are when the direction of vibration of the particles is parallel to the direction of travel
eg sound |
Transverse waves are when the direction of vibration of the particles is perpendicular to the direction of travel
eg water and electromagnetic waves |
Measuring Waves
Displacement, x: the distance of an oscillating particle from its undisturbed/equilibrium position |
Amplitude, a: the maximum displacement from an oscillating particle from its equilibrium position
It is equal to the height of a peak or depth of a trough |
Wavelength, λ: the distance between two consecutive particles at the same phase, measured in metres/m
Example: peak to peak |
Period, T: the time taken for one complete oscillation of a particle in a waves, measured in seconds/s |
Frequency, f: the number of complete oscillations in one second performed by a particle in a wave
note: f =1/ T |
Phase, φ: the point that a particle is at within an oscillation
It can be expressed in terms of an angle up to 360° |
Phase difference, Δφ: the fraction of a cycle between two particles within one or two waves
Example:the top of a peak has a phase difference of half of one cycle compared with the bottom of a trough. |
Phase difference is often expressed as an angle difference. So in the above case the phase difference is 180°. Also with phase difference, angles are usually measured in radians. |
Wave equation
For all waves: |
c = f λ OR speed = frequency x wavelength |
(where speed is in ms^-1 provided frequency is in hertz and wavelength in metres) |
Polarisation
Only TRANSVERSE waves undergo polarisation (where they all travel at the same speed in a vacuum) |
The oscillations within a transverse wave and the direction of travel of the wave define a plane. |
If the wave only occupies one plane the wave is said to be plane polarised. |
Light from a lamp is unpolarised.
However, with a polarising filter it can be plane polarised. |
If two ‘crossed’ filters are used then no light will be transmitted. |
Applications of polarisation
Polarised sunglasses: these contain lenses with polarising filters that only transmit vertically polarised light.
When light is reflected from a reflective surface with water, it undergoes partial plane polarisation (a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane). |
Polaroid photography: work in a similar way to polarised sunglasses.
They are useful for capturing intensified colour and reducing glare on bright days. They also enable photographers to take photos of objects underwater. |
Polarisation of Radio & Microwaves Signals: Radio and television services are broadcast either horizontally- or vertically-polarised.
Therefore, the reception aerial needs to be mounted horizontal or vertical, where its orientation will depend on the transmitter it is pointing towards and the polarity of the services being broadcast. |
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