3: Waves

Question 1

How does Young’s double slit show light is a wave?

Answer 1

-Diffraction and interference are wave properties
-Light has an interference pattern

Question 2

Describe a diffraction grating

Answer 2

-Large number of slits
-Constructive interference causes narrow spots
-Destructive interference causes dark regions
-Commonly used in spectrometers and x-ray crystallography

Question 3

What are the equations for a diffraction grating?

Answer 3

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

Question 4

What are the advantages of optical fibres?

Answer 4

-Signal can carry more information
-No energy is lost as heat
-No electrical interference
-Cheaper
-Very fast

Question 5

What is modal dispersion?

Answer 5

-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

Question 6

What is pulse broadening?

Answer 6

When the receiving signal is wider than the original
-Can cause overlap of signals and information loss

Question 7

How do optical fibres work?

Answer 7

-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

Question 8

What is total internal reflection?

Answer 8

-When the angle of incidence exceeds the critical angle
-Critical angle = arcsin(n2/n1)
-n2 must be less than n1

Question 9

How does light act upon entering a more dense medium?

Answer 9

-It bends towards the normal

Question 10

How is polarisation evidence of transverse waves?

Answer 10

-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

Question 11

Describe longitudinal waves

Answer 11

-Oscillate parallel to propagation of energy
-Refractions are areas of low pressure
-Compressions are areas of high pressure
-Examples are sound waves

Question 12

How are polarisers used in antennas?

Answer 12

-Signals are usually polarised by the orientation of the ariel
-Receiving ariel must be aligned in same plane of polarisation to maximise signal strength

Question 13

What is material dispersion?

Answer 13

-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

Question 14

State snell’s law

Answer 14

n1.Sinθ1 = n2.Sinθ2

Question 15

How do you calculate refractive index?

Answer 15

n = speed in vacuum/speed in material

Question 16

How does white light diffract?

Answer 16

-White central maxima
-Continuous spectra across fringes
-Darker fringes are closer together
-Blue fringes are closer to centre with red fringes furthest from centre

Question 17

State the single slit diffraction equations

Answer 17

w=λ.d/s
Where:
w=fringe spacing
λ=wavelength
d=distance to screen
s=slit seperation

Question 18

Describe double slit diffraction

Answer 18

-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

Question 19

Describe single slit diffraction

Answer 19

-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

Question 20

Define coherence

Answer 20

When waves are of the same frequency

Question 21

Define path difference

Answer 21

Difference in the distance travelled in terms of λ

Question 22

State the harmonic equations

Answer 22

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

Question 23

How are stationary waves formed?

Answer 23

-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

Question 24

Describe transverse waves

Answer 24

-Oscillate perpendicular to propagation of energy
-Examples are electromagnetic waves

Question 25

State the wave equations

Answer 25

-Speed = Frequency times Wavelength
-C=f.λ
-Frequency=1/Period
-f=1/T

Question 26

Define phase

Answer 26

-Point along x-axis on which a particle is observed
-Measured in radians

Question 27

Define phase difference

Answer 27

-Difference in phase angle
-Measure in radians

Question 28

Define speed

Answer 28

-Distance travelled in a given time
-Measured in metres per second

Question 29

Define wavelength

Answer 29

-Distance between two particles in phase
-Measured in metres
-Typically measured peak-to-peak

Question 30

Define frequency

Answer 30

-Number of cycles per unit time
-Measured in hertz

Question 31

Define amplitude

Answer 31

-Maximum displacement from equilibrium position
-Measured in metres

Question 32

Information not on flashcards

Answer 32

-Drawing of stationary waves
-Derivation of n.λ=dSinθ