Y1: Waves Flashcards
What is a Progressive wave
A moving wave
- Carries energy from one place to another without transferring any material
- Caused by an oscillation of particles (or photons) through a medium
What is reflection
A change in direction of the wavefront at an interface between two mediums, so the wave returns to the medium it came from
What is refraction
A wave changes direction as it enters a different medium, due to the wave slowing down or speeding up
What is diffraction
The wave spreads out as it passes through a gap or round an obstacle
What is the displacement of a wave (X)
How far a point on a wave has moved from the equilibrium position (meters)
What is the amplitude of a wave (A)
The maximum magnitude of the displacement (meters)
What is the wavelength of a wave (λ)
The length of one whole wave oscillation or wave cycle, or the distance between two equivalent points such as two adjacent peaks (meters)
What is the period of a wave (T)
Time taken for one whole wave cycle (seconds)
What is the frequency of a wave (f)
The number of whole wave cycles passing a given point, per second (Hertz)
What is the phase of a wave
A measurement of the position of a certain point along a wave cycle (Degrees/radians/seconds)
What is the phase difference of two waves
The phase by which one wave lags behind another (degrees/radians/seconds)
What is the equation for wave speed (c)
c=fλ
What is the value for the speed of light/all EM waves in a vacuum (c)
~ 3x10^8 (ms^-1)
How would you measure the speed of sound
- Use two microphones, with separate inputs to record separate signals, in a straight line ‘d’ meters apart
- Use a signal generator to produce a sound from a loud speaker at one end of the line
- Use a computer to record the phase difference of the who signals (seconds)
- Calculate the speed with d/t
How would you measure the speed of water waves in a ripple tank
- Fill a ripple tank with water at a recorded depth, and use a dipper to create vibrations with regular frequency
- Turn off the main room light and turn on the strobe light above the tank (flashes periodically)
- Increase the strobe light freq. until the waves appear to stand still (wave freq. = strobe freq.)
- Use a ruler to measure the wavelength on white paper beneath the tank (measure multiple and divide)
- Calculate the speed using c=fλ
What is a transverse wave
A wave where the displacement of the particles or field (vibrations) is perpendicular to the direction of energy propagation (transfer)
What are some examples of transverse waves
- All EM waves
- Water
- String vibrations
- S-waves (earthquakes)
What is a longitudinal wave
A wave where the displacement of the particles or field (vibrations) is parallel to the direction of energy propagation (transfer)
What is a polarised wave
A wave that only oscillates along a single plane
Why does no light pass through two perpendicular polarising filters
The first filter will remove all of the vibrations in one directing, and the second filter will remove all the remaining vibrations in the other direction
What happens to the light intensity when there are 3 polarising filters, and the central one is at different angles
0 = Max
45 = 1/4, as each pair reduced it by 1/2
90 = Min
135 = 1/4, as each pair reduced it by 1/2
180 = Max
What are some applications of polarised light
- Glare reduction (Reflected light is partially polarised, so polarisation filters can fully polarise them to remove glare)
- Improving TV/Radio signals (polarised by the orientation of the transmission aerial)
What is the principle of superposition
When two or more waves cross, the resultant displacement is equal to the vector sum of the individual displacements
What is constructive interference
If two waves meet and their displacements are in the same direction, the displacements combine to give a larger displacement
What is destructive interference
If two waves meet with displacements in opposite directions, their displacements cancel as their values are added
What is total destructive interference
If two waves with equal and opposite displacements meet, they are completely cancelled out
What is the phase difference of two waves completely in phase
0 or even multiples π (multiples of 360°)
What is the phase difference of two waves completely out of phase
Odd multiples of π
What is a stationary (standing) wave
The superposition of two progressive waves with the same freq. (or λ) and amplitude, moving in opposite directions.
What is the resonant frequency of a stationary wave
The freq. at which the overall pattern of the stationary wave doesn’t move along, but a whole number of oscillations vibrate up and down
What is the first harmonic of a stationary wave
- A stationary wave oscillating at the lowest resonant freq.
- Total string length = λ/2
- 1 anti-node and a node at each end
What is the second harmonic of a stationary wave
- A stationary wave oscillating at twice the resonant freq.
- Total string length = λ
- 2 anti-nodes and 3 nodes
What is the third harmonic of a stationary wave
- A stationary wave oscillating at three times the resonant freq.
- Total string length = 3λ/2
- 3 anti-nodes and 4 nodes
What are some examples of stationary waves
- Stationary microwaves
- Stationary sound waves
How would you investigate the effect of tension, length and mass per unit length, on the resonant frequency of a wave
- Measure the mass per until length of the whole string
(μ = m/l) - Attach one end of the string to a vibration transducer (and a signal generator) to cause an oscillation.
- Lay the other end of the string over a pulley on the end of the bench, at a distance ‘l’, with a mass attached
- Calculate the tension in the string
(T = mg) - Turn on the signal generator and vary the freq. of the vibration transducer, until you find the first harmonic
- Calculate the frequency
f = 1/2l x √(T/μ)
Change different factors to investigate their effect on this value
How will the mass per unit length of a string effect the resonant frequency
On a heavier string, the wave will travel more slowly, so the frequency will be lower
(f ∝ c)
How will the length of a string effect the resonant frequency
On a longer string, the wavelength will be longer, so the frequency will be lower
(f ∝ 1/λ)
How will the tension of a string effect the resonant frequency
If the tension is higher, the restoring force of each oscillation is greater, so the wave travels faster, and the freq. is higher
(f ∝ c)
How does the size of a slit compared to the wavelength of a wave impact the amount of diffraction
- Slit»_space; λ : No diffraction
- Slit > λ : Some diffraction
- Slit = λ : Maximum diffraction
- Slit < λ :Mostly reflected back
What happens as a wave diffracts around an object
- Causes a shadow, where the wave is blocked
- The wider the obstacle compared to λ, the less diffraction
(ie. can hear around corners, but can’t see)
What is monochromatic light
Light of a single wavelength (colour)
What is coherent light
Light on a single wavelength and frequency with a constant phase difference
Why does a diffraction patter occur if coherent light is passed through a slit
- Bright fringes = Constructive interference
- Dark fringes = Destructive interference
What is light intensity
Power per unit area
For monochromatic light, all photons have the same energy,
∴ increased light intensity = increased photons per second
What happens when white light is diffracted through a single slit
The different wavelengths are diffracted by different amounts creasing a spectrum
What is two source interference
When two monochromatic, coherent wave sources interfere to produce a diffraction pattern
What is Young’s double slit formula for two source interference
W = λD/s
W - Fringe spacing, m (distance between adjacent maxima/minima)
λ - Wavelength, m
D - distance between the slits and the screen, m
s - Slit separation, m
What is a diffraction grating
When light passes through many equally spaced slits (hundreds per mm), causing an interference pattern
What is the diffraction grating equation
dsinθ = nλ
d - distance between slits, m
θ - angle to the normal made by the maxima
n - order of the maxima
λ - wavelength, m
What will happen to the diffraction pattern if you increase the wavelength of light through a diffraction grating
λ ∝ sinθ, so increasing the wavelength will increase θ
This will cause the fringes to spread out
What happens to the diffraction pattern if you increase the distance between the slits in a diffraction grating
d ∝ 1/sinθ, so increasing the distance (less slits per mm) will decrease θ
This will cause the pattern to be less spread out
How can you tell if an order maxima exists in a diffraction pattern
sinθ < 1, so any values of n that cause sinθ > 1 cannot exist
What are some applications of a diffraction grating
- Line absorption/emission spectra (separating the wavelengths of light, used to determine the contents of bodies in space)
- X-ray crystallography (if λ of x-ray ≈ Atom spacing in the crystalline structure, a diffraction pattern will form)
What are the safety precautions when using lasers
- Never shine it towards a person
- Wear laser safety goggles
- Avoid shining at a reflective surface
- Have a warning sign on display
- Turn the laser off when it isn’t needed
What is the absolute refractive index of a material (n)
The ratio between the speed of light in a vacuum (C) and the speed of light in a material (Cs).
(Higher for more optically dense materials)
How do you calculate the absolute refractive index of a material (n)
n = C/Cs
C: Speed of light in Vacuum (=3x10^8)
Cs: Speed of light in material
What is the absolute refractive index of air
1
What is the refractive index between two materials (1n2)
The ratio of the speed of light in material 1 to in material 2
How do you calculate the refractive index between two materials (1n2)
1n2 = n2/n1
What is Snell’s Law of refraction
n1sinθ1 = n2sinθ2
n1 - Refractive index of material 1
θ1 - Angle of incidence
n2 - Refractive index of material 2
θ2 - Angle of refraction
What is the critical angle (θc)
The angle of incidence, for which the angle of refraction is 90, resulting in total internal reflection
Why does total internal reflection occur
If θ1 > θc, refraction cant occur, so all light is reflected back into material 1
How do you calculate the critical angel (θc)
sinθc = n2/n1
What are optical fibres
- Thin, flexible glass tube carrying light signals using total internal reflection.
- Optically dense core, surrounded by optically less dense protective cladding (Step-index optical fibre)
What are some advantages of optical fibres
- Signal can carry more information, as light has a high frequency
- No energy loss, as light doesn’t heat up the fibre
- No electrical interference
- Much cheaper to produce
- Signal can travel a long way very quickly, with minimal signal loss
What is absorption in the signal degradation of optical fibres
Where some of the signal’s energy is absorbed by the material the fibre is made of (causing the amplitude of the signal to decrease)
What is the effect of dispersion in the signal degradation of optical fibres
Pulse broadening, where the same signal is received over a longer time period (causing overlaps leading to signal loss)
What is modal dispersion in optical fibres
A type of dispersions (Pulse broadening), caused by light entering at different angles, causing different amounts of refraction and therefore different speeds
(Reduced by using a single mode fibre, so all rays travel down a narrow path)
What is material dispersion in optical fibres
A type of dispersions (Pulse broadening), caused by different wavelengths of light being refracted by different amounts, due to travelling at different speeds in the material
(Reduced by using monochromatic light)
