3: Waves Flashcards
How does Young’s double slit show light is a wave?
-Diffraction and interference are wave properties
-Light has an interference pattern
Describe a diffraction grating
-Large number of slits
-Constructive interference causes narrow spots
-Destructive interference causes dark regions
-Commonly used in spectrometers and x-ray crystallography
What are the equations for a diffraction grating?
n.λ=d.sinθ
θ=arctan(d/w)
Where n=order
λ=wavelength
d=slit separation=(1*10^-3)/lines per mm
θ=angle between zero order and nth order
What are the advantages of optical fibres?
-Signal can carry more information
-No energy is lost as heat
-No electrical interference
-Cheaper
-Very fast
What is modal dispersion?
-Light takes many different paths
-Thus time of arrival varies
-So two rays sent at the same time can arrive at different times
-Leads to pulse broadening
-Less data is received as a result
-Can be reduced by using a single mode fibre to decrease path difference
What is pulse broadening?
When the receiving signal is wider than the original
-Can cause overlap of signals and information loss
How do optical fibres work?
-Consist of core and cladding
-Core has a high refractive index and cladding has a lower refractive index
-Aim is to maximise total internal reflection
-Cladding protects core from damage
What is total internal reflection?
-When the angle of incidence exceeds the critical angle
-Critical angle = arcsin(n2/n1)
-n2 must be less than n1
How does light act upon entering a more dense medium?
-It bends towards the normal
How is polarisation evidence of transverse waves?
-Polarisation only occurs if a wave’s oscillations are perpendicular to the propagation of energy
-If a wave is passed through a polarisation filter the filter can be rotated to cancel the wave
Describe longitudinal waves
-Oscillate parallel to propagation of energy
-Refractions are areas of low pressure
-Compressions are areas of high pressure
-Examples are sound waves
How are polarisers used in antennas?
-Signals are usually polarised by the orientation of the ariel
-Receiving ariel must be aligned in same plane of polarisation to maximise signal strength
What is material dispersion?
-Different colours of light have different wavelengths and different refractive indexes
-So they have different speeds in the core and arrive at different times
-Colours with smaller refractive index exit first
-This leads to pulse broadening
-Can be solved by using monochromatic light
State snell’s law
n1.Sinθ1 = n2.Sinθ2
How do you calculate refractive index?
n = speed in vacuum/speed in material
How does white light diffract?
-White central maxima
-Continuous spectra across fringes
-Darker fringes are closer together
-Blue fringes are closer to centre with red fringes furthest from centre
State the single slit diffraction equations
w=λ.d/s
Where:
w=fringe spacing
λ=wavelength
d=distance to screen
s=slit seperation
Describe double slit diffraction
-Two sources passed through a double slit
-Waves diffract and cross to superpose
-Constructive interference for 2n.(1/2).λ causes bright fringes
-Destructive interference for 2n+1.(1/2).λ causes dark fringes
-The path difference is d1-d2
Describe single slit diffraction
-Uses a monochromatic laser shone through a single slit
-Waves diffract and travel different distances to screen causing a path difference
-Wave cross and superpose
-Constructive interference causes maxima and bright spots
-Destructive interference causes minima and dark patches
-Intensity of fringes decreases as you go further apart
-Width of central fringe is double the width of other fringes
Define coherence
When waves are of the same frequency
Define path difference
Difference in the distance travelled in terms of λ
State the harmonic equations
1st: λ=2L, f=(1/2L).√(T/µ)
2nd: λ=(2/2)L, f=(2/2L).√(T/µ)
3rd: λ=(2/3)L, f=(3/2L).√(T/µ)
Where:
T=Tension
µ=mass per unit length
How are stationary waves formed?
-A progressive wave travels along a string and is reflected back
-The two waves are travelling in opposing directions and cross and superpose
-Constructive interference causes maxima where amplitudes sum
-Destructive interference causes minima where amplitudes cancel
-Waves must be coherent
Describe transverse waves
-Oscillate perpendicular to propagation of energy
-Examples are electromagnetic waves
State the wave equations
-Speed = Frequency times Wavelength
-C=f.λ
-Frequency=1/Period
-f=1/T
Define phase
-Point along x-axis on which a particle is observed
-Measured in radians
Define phase difference
-Difference in phase angle
-Measure in radians
Define speed
-Distance travelled in a given time
-Measured in metres per second
Define wavelength
-Distance between two particles in phase
-Measured in metres
-Typically measured peak-to-peak
Define frequency
-Number of cycles per unit time
-Measured in hertz
Define amplitude
-Maximum displacement from equilibrium position
-Measured in metres
Information not on flashcards
-Drawing of stationary waves
-Derivation of n.λ=dSinθ
