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Waves
Revision notes for A-Level AQA Physics about topic 3, Waves
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WavesKey Terms
☆ Displacement,
☆ The distance and direction of a particle from the
equilibrium position.
☆ Amplitude,
☆ Maximum displacement of a vibrating particle.
☆ Wavelength,
☆ Shortest distance between two particles in phase.
☆ Frequency,
☆ Number of wave cycles occurring each second.
☆ Wave speed,
☆ Distance travelled by a wave each second.
☆ Phase difference,
☆ Measured in degrees or radians, the amount by which one
wave lags behind another wave.
☆ Path difference,
☆ Measured in metres, the difference in lengths of two waves.
☆ Progressive,
☆ Waves whose oscillations travel and transfer energy.
Longitudinal and Transverse Waves
☆ Transverse
☆ Waves whose oscillations are perpendicular to the direction
of propagation of energy
☆ Longitudinal
☆ Waves whose oscillations are parallel to the direction of
propagation of energy.
☆ Only transverse waves can be polarised, which means all the waves
are oscillating in the same plane.
☆ The discovery of polarised light helped prove that light was a
transverse wave.
Glare and Cameras
☆ Polarisation can be used in things such as polaroid
sunglasses to reduce glare or in a camera to enhance the
image.
Radio Signals
☆ TV and radio signals are polarised by the direction of the
rods on the transmitting aerial.
☆ To receive these signals well, you must ensure the
receiving aerial and the waves are in the same plane.
, Waves
Superposition and Interference
☆ The point where waves meet is called superposition.
☆ The total displacement at a point is equal to the sum of the
individual displacements at that point.
☆ You should know that waves:
☆ Constructively interfere where they are in phase with each
other,
☆ Destructively interfere where they are in antiphase with
each other (180 degrees out of phase).
☆ This can be explained in terms of peaks and troughs.
☆ When the waves are in phase, two peaks or two troughs will
constructively interfere with each resulting in a ‘double’ peak
or trough being created.
☆ When waves are in antiphase, a peak will meet a trough and result
in destructive interference, which is where they cancel each
other out and produce a minimum point.
Stationary Waves
☆ A stationary wave is one that stores energy instead of
transferring it from one point to another.
☆ You need to know the process of a stationary wave being formed on
a string that is fixed at both ends:
1. A wave is generated at one end of the string and travels
down it,
2. At the other end , this wave is reflected and travels back
in the opposite direction,
3. The frequency of wave generation and the length of the
string are such that the next wave generated meets this
reflected wave and undergoes superposition,
4. At places where the two waves are in phase, they undergo
constructive interference and form a maximum point known as
an antinode,
5. At places where the two waves are in antiphase, they
undergo destructive interference and form a minimum point
known as a node.
Waves on a String
☆ The fundamental frequency of a wave on a string can be found from
the following equation:
1 𝑇 𝑀
𝑓 = 2𝑙 µ
𝑇 = 𝑚𝑔 µ = 𝑙
☆ From the equation, we can tell that raising the tension or
shortening the length of a given string increases the pitch.
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