Diffraction Flashcards
Define Path Difference
The difference in the distance travelled by 2 waves
What is a coherent source of waves
Source that emits light waves with constant path difference and the same frequency
What is Monochromatic Light
A Light Wave that is formed of 1 wavelength of light It shows interference the clearest
What is Young’s Double Slit Experiment (YDSE)
Shine monochromatic light from 2 coherent sources with equal width so it diffracts, forming a pattern of light and dark fringes on a screen.
Due to the superposition of waves at the screen
In YDSE, when is a Bright Fringe formed
When 2 light waves meet in phase and superpose Constructive interference occurs
In YDSE, when is a Dark Fringe formed
When 2 light waves meet completely out of phase and superpose
Destructive interference occurs
What is the equation used to calculate the fringe spacing of the pattern formed in young’s double slit experiment
(Wavelength x Dist.) / Slit Seperation
When does destructive interference occur
When one wave has positive displacement and the other has negative displacement
When does constructive interference occur
When 2 waves have displacement in the same direction
What pattern will be produced when white light is used in YDSE
A wider maxima and less intense pattern
Central fringe will be white
Each component colour of white will diffract differently to produce its own fringe pattern which mixes with other fringe patterns
How can one use sound waves for YDSE
Use 2 speakers from the same isgnal genertor so the wave is coherent Use microphone to find intensity of wave to get maximas/minimas
What is Diffraction
Spreading out of waves when they pass through a gap or around an obstacle
How does diffraction vary with the size of the gap
Diffraction is greatest when the gap is the same size as the wavelength
When the gap is smaller, more reflection will occur When the gap is bigger, less diffraction will occur
What happens when monochromatic light is diffracted through a single slit onto a screen
Interference pattern is produced as the wave ‘interferes with itself’ due to Huygens principle
There is a bright central fringe that is double the width of the other fringes Intensity of the fringes decreases the further out from the central fringe
What is a diffraction grating
A slide containing many equally spaced slits very close together
What kind of interference pattern is produced when monochromatic light is shone through a diffraction grating
A brighter, sharper pattern
As there are more slits, more rays interfere with each other
What is X-ray crystallography
Shine an X-ray through a thin crystal sheet since the dist. between atoms is similar to the wavelength of an X-ray Acts as a diffraction grating, forming a pattern that can be used to find atomic spacing in material
Define Refraction
The change of direction that occurs when light passes at an angle across a boundary between 2 transparent substances
Which way does light refract going into a more dense material
Towards the normal
Which way does light refract going into a less dense material
Away from the normal
What is Snells law
sin i/sin r = Refractive Index
is a property of a material, any light ray passing through the same material will have the same refractive index
What is Refractive Index
A measure of optical density
As the angle by which light is refracted is determined by how much the particles in the wave front slow down A more optically dense material will slow down particles more and have a higher refractive index
What is absolute Refractive Index
Ratio between the speed of light in a vacuum and the speed of light in the material n=3x10^8 / speed of light in material
What is relative Refractive Index
Ratio of the speed of light between materials sin x(1) / sin x (2) = n(2) / n(1)
When does Total Internal Reflection occur
When angle i is greater than the critical angle
What is the critical angle
The angle of i when its refracted ray travels along the boundary So angle of refraction is 90
What is the symbol for a Thermistor
What kind of interference pattern is produced when monochromatic light is diffracted through a single slit onto a screen
There is a bright central fringe that is double the width of the others.
Intensity decreases the further out from the central fringe
Equation for the interference pattern produced when monochromatic light is shone through a diffraction grating
dsin (x) = n x Wavelength
n = No. Order
x - Angle to 0 order line
d - Dist. between slits
Equation to use for any refraction question
n(1)sin (x(1)) = n(2)sin(x(2))
n(1) - refractive index of first material
x(1) - angle of incidence
Define Amplitude
A wave’s maximum displacement from the equilibrium position
Define Frequency
The number of complete oscillations passing through a point per second
Define Wavelength
The length of one whole oscillation (e.g. the distance between successive peaks/troughs)
Define Phase Difference
How much a particle/wave lags behind another particle/wave
Define Period
Time taken for one full oscillation
What is a progressive wave
A wave that transfers energy without transferring material
Made up of particles of a medium oscillating
Define a transverse wave
Wave where the oscillation of particles is at right angles to the direction of energy transfer
Define a longitudinal wave
A wave where the oscillation of particles is parallel to the direction of energy transfer
These are made up of compressions and rarefactions and can’t travel in a vacuum.
What is a polarised wave
A wave that only oscillates in one plane
What does a polarised wave provide evidence for
True nature of a transverse wave
Only transverse waves can be polarised as particles oscillate perpendicular to the direction of travel
How do polaroid sunglasses work
Sunlight gets partially reflected on water, roads, shiny surfaces and all the wavelengths therefore line up into one very intense line of polarised light, usually in a horizontal plane
The sunglasses have a filter that polarizes this light, only letting in light in the vertical plane, which is much less intense
How is polaristation used in TV/radio signals
Signals are plane polarised by the orientation of the transmitting ariel
So the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength.
What is superposition
Where the displacements of two waves are combined as they pass each other, the resultant displacement is the vector sum of each wave’s displacement
How is a stationary wave formed
From the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.
What is an antinode, how are they formed
Point of maximum amplitude on a stationary wave
Waves meet in phase and constructive interference occurs
What is a node, when is it formed
Point of no displacement on a stationary wave
Waves meet completely out of phase, total destructive interference occurs
What is the first harmonic
The lowest frequency at which a stationary wave forms
2 nodes and an antinode
How are stationary microwaves formed
By reflecting a microwave beam at a metal plate,
Use a microwave probe to find nodes/antinodes
How are stationary sound waves formed
By placing a speaker at one end of a closed glass tube, lay powder across the bottom of the tube.
It will be shaken at the antinodes and settle at the nodes.
Name I use of Total Internal Reflection
Optical fibres
What is an optical fibre, how do they work
Flexible, thin tubes of plastic or glass which carry information in the form of light signals.
They have an optically dense core surrounded by cladding with a lower optical density to:
Allow TIR to occur
Protects the core from damage
Prevent signal degradation through light escaping the core, which can cause information to be lost.
What can cause signal degradation
Absorption
Modal, Material dispersion = Pulse Broadening
How is signal degradation caused by Absorption
Part of the signal’s energy is absorbed by the fibre, reducing the amplitude of the signal, which could lead to a loss of information
How is signal degradation caused by dispersion
Pulse broadening
The received signal is broader than the original transmitted signal. Broadened signals can overlap causing loss of information.
What is modal dispersion
Pulse broadening caused by light rays entering the fibre at different angles, therefore they take different paths along the fibre
This leads to the rays taking a different amount of time to travel along the fibre
How can modal dispersion be reduced
Making the core very narrow, therefore making the possible difference in path lengths smaller.
Optical fibre repeater - repeats the signal during its travel
What is material dispersion
Pulse broadening caused by light consisting of different wavelengths, meaning light rays will travel at different speeds along the fibre
How can material dispersion be reduced
Using monochromatic light
Optical fibre repeater - repeats the signal during its travel
How to derive dsin (x) = n x Wavelength
Considering the first order maximum, where the path difference between two adjacent rays of light is one wavelength, name the angle between the normal to the grating and the ray of light θ.
As you can see a right angle triangle is formed, with side lengths d and λ. And by using the fact that a right angle is 90°, and angles in a triangle add up to 180°, you can see the upper angle in the triangle is θ.
By using trigonometry we can see that for the first maximum sin θ = , (as sin θ = d λ Opp/Hyp) which rearranges to , (for the first order). dsin θ = λ
We know that the other maxima occur when the path difference between the two rays of light is nλ, where n is an integer, therefore we can generalise the equation by replacing λ with nλ to get:
d sinθ= nλ
Name 2 uses of diffraction gratings
X-Ray Crystallography
Line Absorption spectra
