waves Flashcards
Explain, with reference to refractive index, why the pulse of red light has a shorter transit time than the pulse of blue light
- The refractive index of core for blue light is greater than the refractive index for red
- The speed of the blue light is less than the speed of the red light and travel the same distance
Explain what is meant by modal dispersion in an optical fibre
- Pulse broadening
- Due to different paths through the optical fibre/entering the optical fibre at different angles
- can be prevented by using monomode fibres
Equation to find the wavelength of a stationary wave using length/distance
λ = 2L/n
Equation to find the length of wire used for a stationary wave using the wavelength
L = λn/2
Path Difference
The difference in the distance that to waves have traveled
Phase difference
The amount by which one wave leads or lags behind another measured in degrees or radians
Properties Harmonics
Resonant frequencies occur when an exact number of half wavelengths fit on the string
Resonance f = harmonics
The first harmonic is the lowest possible resonant frequency (f₀)
for the nth Harmonic ->
no. of nodes = n+ 1
no. of anti-nodes = n
frequency = n x f₀
Length =(nλ/2)
wavelength = 2L/n
factors affecting resonant frequencies on a string:
- Length of the string
- extra 1/2 wavelength so lower frequency
- Mass per unit length of the string
- slower travel
- Tension of the string
- slower travel down looser string
F = 1/2π (T/µ)½
Progressive Waves vs Stationary Waves
Progressive Waves
- Amplitude of vibration is the same at every position
- Crests and troughs move along the wave
- All frequencies support progressive waves
- Phase varies continually along the wave
Standing/Stationary Wave
- Amplitude of vibration varies with position
- nodes and anti-nodes do not move along the wave
- only resonant frequencies support stationary waves
- Between adjacent nodes, all points vibrate in phase
ALL KEY WAVE DEFINITIONS
A progressive wave is a mechanism that transfers energy and/or information from one place without the net movement of matter
A mechanical wave is a wave which has oscillations of matter
Longitudinal waves oscillate in the same direction as the wave propagation
Transverse waves oscillate at right angles to the wave propagation
Frequency - the no. of complete oscillations of a wave in one second
Time Period - the time taken to complete one wave cycle
Amplitude of a wave is the max displacement from it’s equilibrium position
wave speed - the speed of which energy is transferred by a wave
Why can radians not be used when doing calculations with practical results involving angles?
You can not convert measured degrees in to radians to calculate angles because it implies a zero level of uncertainty which is not feasibly possible.
Energy of waves at a boundary
The principle of conservation of energy means that, at the boundary between two materials, the energy of the transmitted wave must be equal to the energy of the incident wave.
Diffraction Gratings
- Diffraction Gratings make light fringes brighter and narrower dark fringes which are more intensely dark. Overall making sharper pattern
- With white light it results in a spectrum of colours
Interference
Destructive interference occurs at a path difference of 1/2λ
Constructive interference occurs at path differences multiples of λ
Stationary wave formation & Interference
- The superposition of two progressive waves
- only coherent (waves of a constant phase difference and the same frequency) waves can interfere
-When an oscillator happens to produce an exact no. of waves in the time it takes for a wave to get to the end and back then the two waves reinforce each other
- The frequencies at which this can happen are referred to as the resonant frequencies.
Uses of Polarisers
Light waves that reflect off some surfaces are partially polarised which causes glare that we can filter out using polarising filters.
TV rod signals are polarised by the orientation of the rods on the transmitting aerial
you must align your TV signal aerial to the transmitting aerial or else your signal will be weak because it will be filtered
- same for radio waves
Single Slit
- When a wavelength of light is similar to the size of the slit you get a diffraction pattern
- The central maxima is the brightest part of the pattern because the intensity of the light is the highest at the centre
- For monochromatic light, all photons have the same energy, so this means there is more energy hitting it every second.
- The width of the central maxima varies with the width of the slit and the wavelength of the light
(i.e increase in wavelength = increase in diffraction thus central maximum is wider and intensity is lower
increasing slit width decreases diffraction thus central max is narrower and more intense)
