1.1 Progressive waves distribute energy from a point
source to a surrounding area. They move energy
in the form of vibrating particles or fields.
1.2 There are 2 types of progressive waves
1.2.1 Transverse waves - vibrations are
perpendicular to the wave motion - so
if the wave is travelling horizontally,
the vibrations will be up and down.
1.2.2 Longitudinal waves - vibrations are parallel to
the wave motion - so if the wave is travelling
horizontally, the particles have compressions
and rarefactions. The particle movement is a
series of compressions and rarefactions
1.3 Describing Waves
1.3.1 Amplitude (A) is the maximum displacement
of a particle in a wave from its equilibrium
position. It is measured in metres (m).
1.3.2 Frequency (f) is the number of complete waves passing
a point in one second. It is measured in hertz (Hz).
1.3.3 Wavelength (l) is the distance between two identical points on a
wave (i.e. one full wave). It is measured in metres (m).
1.3.4 Period (T)is the time taken for one complete oscillation of
a particle in the wave. (Distance between two crests on a
displacement/time graph). It is measured in seconds (s)
1.4 Wave speed (c) is measured in
metres per second (ms-1).
1.4.1 wave speed = frequency*wavelength
1.5.1 Phase angle is the position along the
wave, which is normally measured in
degrees or radians. One complete wave is
360 degrees, so from a peak to a trough
will be a change in phase of 180 degrees.
1.5.2 Points on a wave which are always travelling in the same
direction, rising a falling together, are in phase with each other.
1.6 If we measure the distance travelled by two waves and then compare those distances, any difference in the distances
travelled is called the path difference. Path difference is measured in metres (m).
2 LONGITUDINAL AND TRANSVERSE WAVES
2.1 Mechanical waves occur in a medium (solid, liquid or gas).
2.1.1 Use particles to transfer energy
(neighbouring particles bump into each
other and set their neighbours moving).
2.2 Longitudinal waves are waves where the
displacement of the particles in the the same
direction as the wave is travelling in
2.3 Transverse waves are waves where the displacement of the particles in
the medium is perpendicular to the direction the wave is travelling in.
2.4 Electromagnetic waves are oscillating electric an magnetic
fields they include radio waves, microwaves, infra-red,
visible light, ultra-violet, x-rays and gamma rays.
2.4.1 Electromagnetic waves are transverse
waves and they all travel at the speed of
light ( 3 x 108 ms-1 ) in a vacuum.
2.4.2 Don't need particles
2.4.3 Can be reflected, refracted, diffracted and polarised
2.5.1 Plane polarisation is the method of allowing only
waves orientated in the same direction - only
allowing vibrations in one plane.
184.108.40.206 When this light is passed through a polaroid
material only light waves in one plane are
transmited and the light is now polarised.
2.5.2 Polarisation is the orientation of vibration perpendicular
to the direction of energy travel.
2.5.3 Transverse waves can be polarised but
longitudinal waves cannot
220.127.116.11 Longitudinal waves cannot be plane polarised
because the direction of vibration and
direction of propagation are the same
2.5.4 Fishermen wear Polaroid glasses to eliminate reflected glare
from the surface of a lake or stream and they can therefore see
beneath the water more clearly.
3.1 Refractive Index
3.1.1 The refractive index (n) of a material is the ratio
of the speed of light (c) in a vacuum to the
velocity of light in the material (cS).
3.1.2 The refractive index
of a material is always
greater than 1.
3.2 Wavespeed changes when a wave
moves from one medium to another.
That makes the wave refract.
3.2.1 If a light ray speeds up it bends
away from the normal
3.2.2 if a ray slows down it bends
towards the normal
3.3 Snell's Law
3.3.1 Relates to the change in
direction to the change in
speed that takes place.
3.4 Critical Angle
3.4.1 Total Internal Reflection
18.104.22.168 Optical Fibres
22.214.171.124.1 Optical fibres are just 'reflective tubes'. If you
shine light down the tube, and it keeps going
because it bounces from the walls
126.96.36.199.2 Optical fibres are used to carry
signals in the form of pulses of ligh
188.8.131.52.3 Optical fibres are used in medical instruments called
endoscopes and they are used in communications
184.108.40.206 The refractive index of the first
medium is greater than the refractive
index of the second medium (n1>n2)
220.127.116.11 The angle of incidence must be
greater than the critical angle (i>c)
18.104.22.168 Occurs when a propagating wave
strikes a medium boundary at an
angle larger than a particular
critical angle with respect to the
normal to the surface.
3.4.2 When the angle of incidence in water reaches a certain
critical value, the refracted ray lies along the boundary,
having an angle of refraction of 90-degrees. This angle of
incidence is known as the critical angle; it is the largest
angle of incidence for which refraction can still occur.
3.4.3 When light passes from one medium
(material) to another it changes speed. This is
because the speed of a wave is determined by
the medium through which it is passing.
22.214.171.124 When light speeds up as it passes from one material to another,
the angle of refraction is bigger than the angle of incidence.
3.4.4 At any angle of incidence greater than the critical
angle, the light cannot pass through the surface - it
is all reflected.
4 SUPERPOSITION OF WAVES, STATIONARY WAVES
4.1 Superposition of Waves
4.1.1 When two waves pass through the same point they combine
together to either constructively interfere with each other or
destructively interfere with each other before passing on
past each other and continuing their separate journeys
4.2 Constructive Interference
4.2.1 The two waves are in phase with each
other and constructively interfere to
give a wave of greater amplitude.
4.2.2 However the waves will have
4.3 Destructive Interference
4.3.1 The two waves are out of phase (anti-phase) with
each other and destructively interfere to give a
wave of zero amplitude.
4.3.2 The waves in antiphase will cancel each other out.
4.4 Stationary Waves
4.4.1 Stationary waves are formed by two waves with the
same frequency travelling in opposite directions
4.4.2 has parts that remain in a
4.4.3 Nodes == Points of no displacement -
remain in the same position. Particles at
those points are not vibrating at all.
4.4.4 Antinodes == Points on the waveform
where the particles suffer maximum
5.1.1 Two waves are coherent if the phase difference between them is
constant. For this to be the case they must have the same
5.1.2 Two waves are said to be in phase if their crests and troughs meet at the same place at the same
time, and the waves are out of phase if the crests of one meet the troughs of another.
5.1.3 To be coherent:
126.96.36.199 be of the same frequency - monochromatic -
temporally in phase
188.8.131.52 have a constant phase difference - have a
constant spatial phase relationship
184.108.40.206 be of the same amplitude
5.2.1 means having only one wavelength of light
5.3 Interference is the effect of the superposition of waves
5.3.1 Where the 2 crests of the wave meet, causes constructive
interference; but where a crest meets a trough,
destructive interference occurs
5.4 Young's Double Slit Experiment
5.4.1 When laser light passes through a slit it is diffracted. If there
are two slits present the light will diffract at both slits.
5.4.2 The frequency, wavelength and speed of the
waves is not altered.... but the amplitude of the
part of the wave that spreads out is lower - as the
energy is distributed over a bigger area!
5.4.3 If a screen is placed on the other side of the slits from the laser an
interference pattern is seen. It produces a series of bright and dark fringes.
5.4.4 w = fringe spacing in metres (m)
l = wavelength of the light in
metres (m) D = distance between
the double slits and the screen in
metres (m) s = slit separation in
6.1 A normal is the line you draw
perpendicular to the boundary (at right
angles to it). It is drawn so that you can
measure angles from it
7.1 Diffraction happens when a wave hits an obstacle or
gap, diffraction is greatest when the gap is about the
same size as the wavelength of the wave. The waves
bend round the object or spread out when they pass
through the gap, this is called diffraction.
7.1.1 If the gap is small the circular disturbances that get through are
massive compared to the undistrubed wavefronts. Therefore
hardly any of the energy just continues through without change
7.1.2 If the gap is large, compared to the wavelength of the waves passing through ,the
circular disturbances are tiny compared to the undistrubed wavefronts. Therefore most
of the energy just continues through without change.
7.2 Single Slit
7.2.1 When monochromatic laser light is shone through a
narrow single slit a diffraction pattern is produced
consisting of light and dark fringes. It produces a
wide central bright fringe. The other bright fringes
get dimmer as you move away from the centre.
7.3 Diffraction Grating
7.3.1 A diffraction grating is a piece of glass with lots of closely
spaced parallel lines on it each of which allows light to pass
through it, this is a transmission diffraction grating.
7.3.2 Diffraction gratings are used in spectrometers. The diffraction grating
splits up the light into a spectra.
7.3.3 d = grating spacing in metres (m) J = angle of
diffraction n = order number l = wavelength on
the light in metres (m)