Waves Flashcards
What is a progressive wave?
A wave that transfers energy without transferring material
What is amplitude?
A waves maximum displacement from the rest position
What is frequency?
Number if complete oscillations passing a point per second
What is wavelength?
Distance from peak to peak or tough to trough
What is speed in a wave?
Distance travelled by the wave per unit time
What does phase mean in waves?
The position of a point on a wave cycle
What is phase difference?
How much a particle/ wave leaves behind another particle/wave
What is time period?
The time taken for one full oscillation
What does it mean when two points in a wave are “in phase”?
When they are both at the same point in a wave cycle
Have the same displacement and velocity
Phase difference is a multiple of 360°(2pi radians)
What does it mean when two points in a wave are “out of phase”?
They are an odd integer cycles apart
What is a transverse wave?
Oscillations of particles are at a right angle to the direction of energy transfer
How fast do electromagnetic waves travel in a vacuum?
3x10^8 m/s
They’re transverse
What are longitudinal waves?
Oscillation of particles is parallel to the direction of energy transfer
What are longitudinal waves made up off?
Compressions and refractions
What can’t longitudinal waves travel through?
A vacuum
How does a polarised wave oscillate?
In one plane
(Up and down)
How does polarisation provide evidence for the nature of transverse waves?
Polarisation can only occur if a waves oscillations are perpendicular to its direction of travel (transverse waves)
How do Polaroid sunglasses work with polarisation?
They reduce glare by blocking partially polarised light reflected from water and tarmac
Only allow oscillations of the plane of the filter making it easy to see
How do tvs and radio signals use polarisation?
They are plane polarised by the orientation of the rods transmitting the aerial
Receiving aerial must be in the same plane of polarisation to receive the signal at full strength
What is superposition?
When the displacement of two waves are combined as they pass eachother, the resultant displacement is the vector sum of each waves displacement.
Has two types of interferences
What is constructive interference?
Occurs when two waves have the same displacement in the same direction
What is destructive interference?
Occurs when one wave has a positive displacement and the other has a negative displacement
If two waves have equal but opposite displacements total destructive interference occurs
How is a stationary wave formed?
From the superposition of 2 progressive waves travelling in opposite directions in the same plane
The waves have the same frequency, wavelength and amplitude.
What happens where waves meet in phase?
Constructive interference occurs so antinodes are formed which are regions of maximum amplitude.
What happens where waves meet out of phase?
Destructive interference occurs and nodes are formed which are regions of no displacement
What is the first harmonic?
Lowest frequency at which a stationary wave forms
Forms a stationary wave with two nodes and a single anti node
Distance between adjacent nodes or antinodes is half a wavelength
How can you use the first harmonic to find the second and third harmonic?
Double it to find the second(2 antiondes)
Triple it to find the third(3 antinodes)
How can stationary microwaves be formed?
By reflecting a microwave beam at a metal plate
To find the nodes and antinodes use a microwave probe
How can stationary sound waves be formed?
By placing a speaker at one end of a closed glass tube, lay power across the bottom of a tube, it will be shaken at the antinodes and settle at the nodes
What is path interference?
The difference in distance travelled by two waves
What is a coherent light sources features?
Same frequency and wavelength
Fixed phase difference
What is Young’s double slit experiment?
Shine a coherent light through two slits about the same wavelength of the laser so light diffracts
Light fingers are formed where the light meets in phase and interferes constructively, the path difference between waves is a whole number (n wavelengths)
Dark fringes are formed where the light meets completely out of phase and interferes destructively, the path difference is a whole number and half wavelengths (n+0.5 wavelengths)
What would happen if you used a white light instead of a monochromatic laser?
Gives wider maxima and less intense diffraction pattern
Central white fringe
Alternating bright fringes which are spectra, violet closest to central maximum and red furthest
How is evidence for wave of natural light provided by young’s double slit experiment?
Because diffraction and interference are wave properties, and so proved that EM radiation must act as a wave.
What is diffraction?
Spreading out of waves when they pass through or around a gap
When does the greatest diffraction occur?
When gap is same size as wavelength
When does less diffraction occur?
The wider the obstacle compared to wavelength the less diffraction
Why do we get a spectrum of colour of white light?
White light is made up of all colours and therefore different wavelengths of visible light which are all diffracted by different amounts
What does increasing slit width do?
Decreases the amount of diffraction so rhe central maximum becomes narrower and intensity slightly increases
What does increasing the lights wavelength in diffraction do?
Increases the amount of diffraction as slit is closer in size to lights wavelength, therefore central maximum becomes wider and intensity increases
What is a diffraction grating?
A slide containing many equally spaced slits close together
Why is passing light through a diffraction grating better?
The interference pattern is much sharper and brighter because there are more rays of light reinforcing the pattern, meaning slit measurements are more accurate and easy to take
How do we derive the formula d(sin theta) = n (wavelength)
Considering first order maximum where path difference between two adjacent rays of light is one wavelength and the angle between normal to the grating and ray of light is theta
Right angle triangle is formed with side lengths d and wavelength and using fact right angle is 90° and angles in triangle add up to 180° you can see upper angle in triangle is theta
Using trigonometry we can see that for first maximum (sin theta) = wavelength / d which you can rearrange
We know maxima occurs when path difference between two rays of light is n (wavelength), where n is an integer so we replace wavelength with n wavelength to get d (sin theta) = n wavelength
What does splitting up stars with diffraction gratings do?
You get the absorption spectra which can be used to shows which elements are present in a star
How does x ray crystallography use diffraction gratings?
X rays are directed at thin crystal sheet which acts as a diffraction grating to form a diffraction pattern.
Happens because wavelengths of x rays are similar to size in gaps between atoms.
Can be used to measure atomic spacing in certain materials.
What is a refractive index?
Property of a material which can measure how much it slows down light passing through it
What does it mean when a material is optically dense?
It has a high refractive index
When does refraction occur?
When a wave enters a different medium causing it to change direction either towards or away from the normal depending on the materials refractive index
What happens as light moves across boundry of two materials?
It’s speed changes which causes its direction to change
What happens if n2 is more dense than n1?
Ray of light slows down and bends towards the normal
What happens if n2 is less dense than n1?
Ray of light will bend away from the normal
What is total internal reflection?
When the angle of incidence is greater than the critical angle
The incident refractive index is greater than the refractive index of the material at the boundary
What is the critical angle?
When angle of refraction is exactly 90° and the light is reflected along the boundary
How do optical fibres use total internal reflection?
They have an optically dense core surrounded by cladding with a lower optical density allowing TIR to occur
This cladding also protects core from damage and prevents signal degrading through light escaping the core which can cause information to be lost
How does absorption causes signal degrading?
Part of the signals energy is absorbed by the fibre reducing the amplitude of the signal which could lead to a loss of information
How does dispersion causes signal signal degrading?
Causes pulse broadening which is where received signal is broader than the original transmitted signal
Broadened signals can cause overlap causing loss of information.
What is modal dispersion?
Caused by light rays entering the fibre at different angles therefore they take different paths, leading to the rays taking a different amount if time to travel along fibre casing pulse broadening.
How can we reduce modal dispersion?
By making core very narrow therefore making the possible difference in path lengths smaller
What is material dispersion?
Caused by using light consisting of different wavelengths meaning light rays will travel at different speeds along the fibre which leads to pulse broadening
How can we prevent material dispersion?
Using monochromatic light
How can both absorption and dispersion be reduced?
Using an optical fibre repeater which regenerates the signal during its travel to the destination
What is optical density?
The ability of a material to transport and emit EM waves
