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Waves (Waves and vibrations (Polarisation (• Transverse waves plane…
Waves
Waves and vibrations
Mechanical waves
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• -Examples include seismic waves, sound waves and waves on strings
• -When these waves pass through substances the particles of the substance vibrate which causes other particles to vibrate in the same way
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Polarisation
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• If unpolarised light passed through 2 polaroid filters, then light intensity changes depending on how one filter is turned relative to the other
• Then one filter perpendicular to other no light can pass through as light not travelling in the same plane
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Measuring waves
Key terms
• The displacement of a vibrating particle is its distance and direction from its equilibrium position
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• Wavelength (λ) least distance between two adjacent vibrating particles with same displacement and velocity at same time
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• Frequency is number of cycles per second (or number of complete waves passing a fixed point per second)
Wave speed
• Higher frequency of a wave, the shorter its λ
• Speed of waves (c) = frequency (f) × wavelength (λ)
Phase difference
• Phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the cycle started
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Wave properties
Ripple tanks
• Used to observe properties such as reflection, refraction and diffraction
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Reflection
• Straight waves directed at a hard flat surface reflect off at the same angle as they were directed at (angle of incidence = angle of reflection)
• Angle between incident ray and plane mirror is equal to the angle between the reflected ray and the mirror
Refraction
• When waves pass across a boundary to material of different density the speed changes so λ changes (freq stays constant)
• If wavefronts approach at an angle, then they change direction – this is refraction
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More wave properties
Superposition
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• Principle of superposition states that when two waves meet, total displacement at a point is equal to the sum of the individual displacements at that point
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• When a crest and a trough of same amplitude meet, they cancel out (resultant displacement of 0)
• Interference is where waves of constant freq and constant phase difference cancel and reinforce at fixed points
• Coherent sources of waves produce interference pattern where they overlap as same freq waves with constant phase difference
• If phase difference randomly changed, points of cancellation and reinforcement would change at random and no interference pattern seen
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