Waves Flashcards
Define amplitude
The maximum displacement of the wave from its equilibrium position
What is phase difference and what is it measured in?
How much a particle/wave lags behind another. Measured in radians, degrees or fractions of a cycle
What does a polarising filter do? also explain what partially polarised means.
Only allows oscillations in one plane. if you have two at right angles to each other no light will get through. Light can become partially polarised when reflected from some surfaces as some of it vibrates in the same direction. So viewing partially polarised light through a polarising filter at the correct angle you can block out unwanted glare.
How is polarisation used as evidence of the nature of transverse waves?
Polarisation can only occur if a waves oscillations are perpendicular to its direction of travel
What is a stationary wave?
A wave which transfers no energy and positions of maximum and minimum amplitude are constant
What is a node and anti node
Node: point on a stationary wave where displacement is 0
Anti node: a point on a stationary wave with maximum displacement.
What are the conditions required for a stationary wave to be produced?
The waves must be the same frequency, wavelength and amplitude.
Must be travelling in opposite directions
When the waves meet in phase constructive interference occurs so antinodes form.
Where waves meet completely out of phase, destructive interference occur and nodes form.
What is the principle of superposition?
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.
Describe the first harmonic of a stationary wave with two closed ends.
2 nodes at either end and an anti node in the middle.
Define coherent
If they have the same wavelength, frequency and a fixed phase-difference apart.
Constructive interference occurs when path difference is…
Destructive interference occurs when path difference is…
nλ
(n+1/2) λ
How do you demonstrate stationary waves with microwaves and sounds?
Microwaves reflected off a metal plate set up a stationary wave. You can find the nodes and antinodes by moving a probe between the transmitter and the reflecting plate.
Powder can show stationary waves in a tube of air. Stationary sound waves are produced in the glass tube. A powder laid along the bottom of the tube are shaken away from the antinodes but left undisturbed at the nodes.
How do you I work out the mass per unit length? AKA μ
μ=M/L
M=mass of string
L=length of string
Why is a laser useful in showing interference and diffraction?
It produces monochromatic light so diffraction and interference patterns are more defined.
What was young’s double slit experiment?
A single light source is directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.
Describe the interference pattern created when using white light.
A bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red the furthest away.
Why does an interference pattern form from when light is passed through a single slit?
The light diffracts as it passes through the slit, where the waves are in phase constructive interference occurs making bright fringes and where the waves are completely out of phase destructive interference occurs making a dark fringe.
What happens to the central maximum when you increase the width of the slit?
The central maximum becomes narrower, but the intensity of the central maximum increases.
What happens when you increases the wavelength in diffraction?
Increases the amount of diffraction meaning the central maximum is wider and the intensity of central maximum has decreased.
What do the symbols in fringe spacing equation represent?
w=fringe spacing
s=spacing between slits
D=distance from splits to screen
How would i see the interference patterns of microwaves?
Replace laser and slits with two microwave transmitter cones attached to the same signal generator. Also replace screen with a microwave receiver probe. If you move the probe perpendicular to the direction at which the microwave generators are facing you will get an alternating pattern of strong and weak signals just like the light and dark fringes on the screen.
Diffraction patter when monochromatic light is shone through a narrow slit.
Central maximum, with dark and bright fringes alternating on either side. Dark and bright fringes caused by destructive and constructive interference.
What happens when you increases the number of slits in a diffraction grating?
The interference pattern becomes very sharp because there are so many beams reinforcing the pattern. Sharper fringes make for more accurate measurements.
describe monochromatic light on a diffraction grating.
All maxima are sharp lines.
Line of maximum brightness in the middle called zero order line.
Line either side of the central one are called first order lines, the next pair out are called second order lines and so on.
Derive the diffraction grating equation
At each slit the incoming waves are diffracted. These diffracted waves then interfere with each other to produce an interference pattern.
d=the distance between two slits (slit spacing)
theta=the angle between the order maximum and the incoming light.
Lambda=the path difference.
Therefore dsintheta=lambda
The other maxima occur when the path difference is 2lambda, 3lambda
List some applications of diffraction gratings
X-ray crystallography
Crystal sheet acts as the diffraction grating the x-rays pass through, used to find the spacing between atoms.
Splitting up light from stars to make the line apsorption spectra. Used to identify elements present in stars.
When does total internal reflection occur?
When light goes from an optically denser medium into a optically less dense medium
As the light bends away from the normal, when you increase the angle of incidence the angle of refraction gets closer and closer to 90degrees. Eventually the angle of incidence reaches a critical angle where the angle of refraction is 90degrees and the light is refracted along the boundary. When the angle of incidence is greater than the critical angle more refraction is impossible so all the light is reflected back into the material.
What is the purpose of cladding in a n optical fibre?
Protects the core from scratches, which would allow light o escape, degrading the signal
Allows for TIR as it has a lower refractive index than the core.
How does signal degradation by absorption in a n optical fibre affect the received signal?
Some of the signals energy is lost through absorption by the material the fibre is made from. This energy loss results in the amplitude of the signal being reduced.
what does modal and material dispersion cause? And how do we resolve this? How can resolve both problems at once?
Modal dispersion - light rays enter at different angles and so take different paths. Some take longer paths some take shorter causing pulse broadening. Resolve this by making optical fibre tube narrower
Material dispersion - When light with different wavelengths travel at different speeds in the fibre. This causes pulse broadening. Can be resolved by using monochromatic light instead.
To resolve both problems at once use an optical fibre repeater to regenerate the signal through the fibre.
