# Waves

Everything from the waves section of the edexcel spec

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Physics Paper 2 (Waves)
Some waves require a medium to travel through. These are called mechanical waves. Electromagnetic waves require no medium to travel
through.
In all waves, something oscillates in order to transfer energy. In transverse waves, the oscillations are perpendicular to the direction of
movement. In longitudinal waves the direction of oscillation is parallel to the direction of movement.
The direction of travel is known as the direction of propagation of the wave.
Circular waves appear as ripples in a circular motion from a source. Plane waves appear as a series of parallel lines moving in a continuous
direction. A Pulse does not contain any repeated up or down motion. Wave trains are sections of a wave with a beginning and an end.
Physicists refer to continuous waves to demonstrate principles and ideas. They have no beginning or end.
The wave front is a line on a wave where all parts are at the same phase ­ usually the crest. Rays are lines that show the direction of movement
from the disturbance and are 90 degrees to the wave front.
The velocity of a wave is linked to its wavelength and frequency. The wavelength is the distance between any 2 similar points on a wave. The
frequency is a measure of how many waves pass a point in 1 second.
v = f = 1
T where T is the time for 1 wave cycle and thus f = T .
Amplitude is the maximum displacement of any point on a wave. The equilibrium position is the center of the amplitudes.
Waves are created by an oscillation. Points on a wave that are apart oscillate in phase and points that are /2 oscillate in antiphase. To
describe this, we talk about one being 360°.
The principle of superposition states that where 2 or more waves meet, the total displacement at any point is the sum of the displacements that
each individual waves at that point. This would cause waves that are in phase to `add up' and waves that are in antiphase to `cancel each other
out'.
The phase position is dependent on how far the wave has travelled and superposition thus depends on path difference.
Progressive waves are those in which the peaks are troughs are moving. Stationary waves are those, which the position of the peaks and
troughs are not moving.
Stationary waves are created when 2 waves interfere constructively and destructively to add up and antinodes and to cancel out at nodes.
For a string stretched between two points gas a fundamental frequency f0 and = 2 l 2l
1 . In the second harmonic, 2f0, = 2 and there are 2
2l
antinodes. In the third harmonic, 3f0, there are 3 antinodes and = 3 .
When a wave is reflected at the end of a string, it reflects with a 180degree phase change.
To calculate the frequency of the harmonic for a stretched string, the formula is used:
f=2
n
l
T
Where f=frequency, n=harmonic, T=tension in string and =mass per unit length e.g. kg/m.
Huygen's construction is an explanation for the way that a circular wave spreads out leading to a plane wave and the radius becomes very large.
In reflection, the angle of incidence=the angle of reflection. This is known as the law of reflection. The angle is measured from the normal,
which is perpendicular from the surface.
In refraction: from less dense to more dence= towards the normal and from more dense to less dense = away from the normal. The angle of
refraction is dependent on the refractive index:
sin i = = speed in medium 1
sin r speed in medium 2
This is known as Snell's law.
As the angle of incidence gradually increases, the angle of refraction becomes larger. At the point when this is 90degrees, the angle of incidence is
known as the critical angle. As the incidence angle surpasses this, only reflection will occur (total internal reflection).
When a wave passes through a gap or round an object it will deviate from its path. This is known as diffraction. The amount of diffraction is
dependent on the ratio between the wavelength and the size of the gap. More diffraction occurs when the size of the gap is similar to the
wavelength.
Interference occurs when waves overlap each other to produce a patter where waves reinforce each other and where they cancel each other out.
When light is diffracted from 2 different slits, they will interfere on a screen to form light and dark patches. Light patches are formed when the
waves are in phase and such that the path difference is a whole number of wavelengths different:
path difference
= whole number
Dark patches occur when the path difference is a whole number of half wavelengths, so that the waves are 180 out of phase.
2 x path difference
= whole number
This is only true of the light is from a coherent light source and therefore the light leaves the slits at the same wave phase.

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Polarization is where waves are made to oscillate in one plane only. This only happens to transverse waves as they can oscillate in multiple
planes. Waves can be polarized to avoid interference.
As all electromagnetic waves travel at the same speed, frequency and wavelength are inversely proportional: as one gets big the other gets small:
8
3x10 = f
A moving object emitting waves will not produce waves of the same wavelength. This is known as the Doppler effect.…read more