Waves

Question 1

Fourier?

Answer 1

All waveforms can be described by superposition of sine or cosine waves.

Question 2

Explain phase difference

Answer 2

How out of sync waves are.

Pi phase difference is half a wavelength and means the waves are completely out of phase. E.g. Meeting trough to crest

Question 3

Stationary wave

Answer 3

Wave reflected back towards itself.

Interference with itself creates areas of no disturbance (nodes) and areas of maximum disturbance (antinodes)

Question 4

Derive Doppler effect for stationary observer

Answer 4

Frequency is waves output in one second. They normally occupy the distance in one second (speed of sound waves)

But this distance changes when source is moving. Wavelength is now (v+v)/f

Substitute into f=v/lamda

Question 5

Doppler effect moving observer

Answer 5

Now the wavelength doesn’t change

Velocity does change!

So f=(v+v)/lamda

Question 6

Conditions or coherent light

Answer 6

Constant phase difference

Same frequency and wavelength

Question 7

Why is light more difficult to make coherent?

Answer 7

Random nature of electron transitions and photon emission

Question 8

Define optical path difference and relate to phase difference

Answer 8

Geometric distance multiplied by refractive index of medium.

If opd is whole no of wavelength then in phase. If its a whole no of half wavelengths then pi out of phase.

Question 9

Division of Amplitude??

Answer 9

Beam of light split into two by reflection and transmission.

Then two beams later recombined to create interference

Question 10

Why can you use extended sources of light for division of amplitude?

Answer 10

Individual beams of light interfere with themselves. Doesn’t have to be coherent

Question 11

Reflection from high to low refractive index??

Answer 11

No phase chane

Question 12

Reflection from low to high refractive index??

Answer 12

Pi phase change

Question 13

Derive thin film interference for constructive(maxima) and destructive (minima)

Answer 13

Destructive
OPD = (0.5+m) lamda

Constructive
OPD = mLamda

Remember the phase changes!!

Question 14

Thin wedge derivation!

Answer 14

OPD = 2t + pi phase change (0.5lamda)

When destructive
(0.5+m)lamda= 2t+(0.5lamda)

Substitute t for x and d and l

Work it out

X= (lengthmlamda)/(2*thickness)

Now get next minima along x’
(When m=m+1)

Now find change between these two x values and get rid of fluff on RHS.

X=(length * lamda) / (2d)

Question 15

Thickness for non-reflective coating derivation!!

Answer 15

OPD is 2nd
Destructive therefore

2nd= (m+0.5) lamda

Re-arrange for d.

M= 0 is thinnest

Question 16

Division of Wavefront definition

Answer 16

Single slit creates coherent wavefronts from mono light source

Double slit creates divides light into two wavefronts that now interfere with each other.

Question 17

Derivation of

ChangeX=(lamda*D)/d

Answer 17

Compare triangles in diagram. Show path difference between light creating maxima from s1 and s2.

Assume because angle is so small

Question 18

Transverse waves and polarisation

Answer 18

Ahh. Only transverse ones can be polarised.

Question 19

Derive brewsters angle!

Answer 19

N= sin(incidence) / sin(refraction)

At brewsters the angle between refraction and reflection is 90 Degrees.
Draw a goddam diagram and do it.

Sin over cos is tan

Question 20

Explain Coloured Fringes from thin film

Answer 20

Produced by interference by division of amplitude.

Different colours produced from
Angle of viewing
Thickness of film
Optical path difference