Definitions 3 Flashcards
Progressive waves
Waves that transfer energy, but not matter, as a result of oscillations or vibrations through a medium or vacuum
Transverse waves
Waves in which the particles oscillate perpendicular to the direction of motion and energy transfer
Longitudinal waves
Waves in which the particles oscillate parallel to the direction of motion and energy transfer
Frequency
The number of oscillations of a wave per unit time
Period
The time taken for one complete oscillation of a wave
Amplitude
The maximum distance of a particle in a wave from its equilibrium or rest position
Displacement
The distance of a point on a wave from its rest or equilibrium position
Phase difference
The difference in positions of two points on a single wave or between two waves of the same frequency or how much a point or a wave is in front or behind another
Intensity
The power per unit area or the energy passing through a unit area per unit time
Doppler effect
The change in observed frequency when a source of sound waves moves relative to a stationary observer
Electromagnetic waves
Transverse waves that travel at the speed of light through a vacuum
Polarisation
The restriction of the oscillations of the particles in a transverse wave to only one direction
What are the properties of standing waves?
- They are formed from waves of the same frequency, amplitude and wavelength travelling in opposite directions
- They consist of nodes and antinodes that are fixed - the peaks and troughs do not move
- They do not transfer energy
- All points between two adjacent nodes oscillate in phase
- All points in adjacent loops oscillate in anti-phase
Wavelength
The distance between two points in phase with each other on consecutive oscillations of a wave
What happens when the time-base unit or voltage unit on a cathode-ray oscilloscope turns off?
- If the time-base unit is turned off you will see a straight vertical line on the screen
- If the voltage unit is turned off you will see a straight horizontal line on the screen
What is the relationship between the frequency and wavelength of a wave?
For a wave at constant speed, λ ∝ 1 / f, so when λ increases, f decreases and vice versa
State the relationships between the intensity, amplitude and frequency of a wave?
- I ∝ A^2
- I ∝ f^2
Hence, if A or f is doubled, I increases by a factor of 4 or 2^2
What is the relationship between the intensity and distance of a spherical wave?
The area of a spherical wave passes through the surface area of a sphere, so A = 4πr^2, hence:
I = P / 4πr^2
If no energy is absorbed, we get:
I ∝ 1 / r^2
As you move farther from the source the same energy gets spread over a larger area, so the intensity of the spherical wave decreases rapidly
Why can’t longitudinal waves be polarised?
The particles of a longitudinal wave oscillate parallel to the direction of energy transfer, which means they are already moving one direction
When occurs when unpolarised light meets a polariser?
The intensity of the unpolarised light will decrease be 1/2 when it moves through the polariser
What occurs if the polariser and analyser have the same orientation?
If they have the same orientation, the transmission axes of both filters are 0° or 180° to each other, which means that the incident light will have the same intensity as the transmitted light, since cos(0° or 180°) = 1
What if the polariser and analyser are at right angles?
If they are at right angles, the transmission axes of both filters are 90° or 270° to each other, and so as cos(90° or 270°) = 1, the intensity of the transmitted by the analyser will be zero
State the equation for polarisation:
I = Io * cos^2(θ)
θ = the angle between the direction of the incident light and the transmission axis of the polariser
What is the formula for the length of a stationary wave that is fixed at both ends?
L = nλ / 2
n = all positive integers
What is the formula for the length of a stationary wave that is open at one end?
L = nλ / 4
n = only odd positive integers
Why do stationary waves that are open at one end only have odd harmonics?
There is always a node at the closed end of the stationary wave and an antinode at the open end, which causes only odd multiples of 1/4λ to be added to each successive harmonic
What is the formula for the length of a stationary wave that is open at both ends?
L = nλ / 2
n = all positive integers
What happens to a wave when it is diffracted?
The amplitude decreases, but all other properties remain the same
Path difference
The difference in distance one wave has to travel compared to another to reach the same point
How do you calculate the path difference for constructive interference?
Constructive interference = nλ
n = the maxima you are looking to calculate (0…5)
How do you calculate the path difference for deconstructive interference?
Deconstructive interference = (n +1/2)λ
n = the minima you are looking to calculate (0…5)
Diffraction grating
A diffraction grating is a plate on which there is a very large number of closely-spaced, identical slits so that when monochromatic light is incident on it, a pattern of narrow bright fringes is produced
How do you calculate the angle between a maxima and the central maxima?
θ = tan-1(h / D)
h = n * x - distance between maxima
D = distance between slits to screen
What do bright fringes look like?
They are bright bands of light that are evenly spaced out and their intensity decreases from either side of the central bright fringe
How do you identify areas of compression and rarefaction on a displacement time graph?
Areas of compression and rarefaction always appear when x = 0
How do you identify an area of compression?
If the displacements above and below the point are coming towards each other, the point is an area of compression
How do you identify an area of rarefaction?
If the displacements above and below the point are moving away from each other, the point is an area of rarefaction
List the conditions for superposition:
- The waves must be of the same type - both transverse or longitudinal
- The waves must be coherent - have the same frequency and constant phase difference
- The amplitude only needs to be the same on both waves to form a stationary wave
Monochromatic
Light of a single wavelength
How do you calculate phase difference
What is a cathode-ray oscilloscope?
An instrument that displays a voltage against time graph for an electric circuit
Derive the equation v = fλ:
- The speed of a wave is given by s = d/t
- A wave travels one wavelength on one period, hence, v = λ/T
- If you substitute f = 1/T, the equation becomes v = fλ
How do you calculate the intensity of a wave?
I = P/A
Where, A is the area perpendicular to the energy transfer of the wave
What are the properties of transverse waves?
- They have crests and troughs
- They can travel through a vacuum
- They can be polarised
What are the properties of longitudinal waves?
- They have areas of rarefactions and compressions
- They cannot travel through a vacuum
- They cannot be polarised
What are areas of rarefaction and compression?
- A rarefaction is an area of low pressure, with the particles being further apart from each other
- A compression is an area of high pressure, with the particles being closer to each other
What happens when a source of sound waves moves closer to an observer?
The wavelength of the sound waves shorten and compress, which therefore increases the observed frequency for the observer
What happens when a source of sound waves moves away from an observer?
The wavelength of the sound waves broaden, which therefore decreases the observed frequency for the observer
State the equation for the Doppler effect:
fo = fs [ v / ( v ± vs ) ]
v = 340ms^-1
What is the relationship between the frequency and energy of an EM wave?
The higher the frequency, the more energy that an EM wave possess, making more ionising, which can lead to cancer
Superposition
When two or more waves arrive at the same point and overlap, their amplitudes combine
The principle of superposition
When two or more waves overlap at a point, the displacement at that point is equal to the sum of the displacements of the individual waves
Stationary waves
Stationary waves are produced by the superposition of two waves of the same frequency, amplitude and wavelength travelling in opposite directions
Nodes
Nodes are where there is no vibration, they are always fixed
Antinodes
Antinodes are where the vibrations are at their maximum amplitude, they only move in the vertical direction
How do you change the harmonic of a stationary wave?
Increasing the frequency increases the wavelength and the number of nodes and antinodes present, while decreasing the frequency does the opposite
What is diffraction affected by? DONT UNDERSTAND
Diffraction is dependent on the width of the gap compared to the wavelength of the waves:
- For gaps that are much smaller than the wavelength, no diffraction occurs
- For gaps that are much bigger than the wavelength, no diffraction occurs
When the wavelength of the wave and the width of the gap are similar in size, then diffraction occurs:
- When the wavelength is bigger than the gap, more diffraction occurs
- When the wavelength is smaller than the gap, less diffraction occurs
Diffraction is the most prominent when the width of the slit is approximately equal to the wavelength
Inteference
Interference, which results from the principle of superposition, occurs when waves overlap and their resultant displacement is the sum of the displacements of each wave
Constructive and deconstructive interference
- Constructive interference occurs when the overlapping waves are in phase, producing a resultant wave that has double the amplitude
- Deconstructive interference occurs when the overlapping waves are in anti-phase, resulting in a resultant wave that has no amplitude
Coherence
Two or more waves are coherent with each other if they have the same frequency and constant phase difference
What are the two types of interference fringes?
- Bright fringes are an indication of an area of constructive interference
- Dark fringes are an indication of an area of deconstructive interference
What are the requirements for two source interference?
- The sources must be coherent
- The sources must be monochromatic
Describe the setup of Young’s double slit experiment:
- There may be two sources, which are made to interfere with each other
- There may be a single source, which is made to pass through two small slits
What is Young’s double slit equation?
λ = ax / D
a = distance between the centres of the slits in m
x = distance between two adjacent maxima or minima in m
D = distance between the slits and the screen in m
What is the diffraction grating equation?
dsin(0) = nλ
d = distance between two adjacent slits in m
θ = angular separation between the order of maxima in °
n = order of maxima (0…5)
What is represented by N in diffraction gratings?
N is the number of lines per mm,m or cm:
d = 1 / N
Where is the maximum angle of diffraction?
The maximum angle of diffraction occurs when θ = 90° and hence, sin θ = 1
How do you find the highest order of maxima?
The highest order of maxima is given by n = d / λ, when θ = 90° and hence, sin θ = 1, since n must be a positive integer, if the value obtained is a decimal it must be rounded down
