M4 Waves - Part 2 Flashcards

superposition, young's slits, path difference, diffraction gratings

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1
Q

What is the principle of superposition of waves

A

when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

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2
Q

what is constructive interference

A

when 2 waves meet, if their displacements are in the same direction, they combine to give a bigger displacement
the points are known as maxima

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3
Q

what is destructive interference

A

if a wave with a positive displacement meets a wave with a negative displacement, they cancel each other out
when it’s equal and opposite displacements they completely cancel out (total destructive interference)
the points are known as minima

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4
Q

what is coherence

A

waves are emitted from 2 sources with a constant phase difference

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5
Q

what does it mean if waves are coherent

A

a constant phase difference/relationship

same frequency and wavelength

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6
Q

what do the waves have to be for interference occur

A

when two waves are coherent

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7
Q

what are examples of interference

A
water waves
sound waves ( 2 speakers connected to same signal generator)
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8
Q

what is the equation for path difference in constructive interference

A

path difference =nλ

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9
Q

what is the equation for path difference in destructive interference

A

path difference =(n+0.5)λ

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10
Q

in interference, what is it called when the path difference is 0

A

central maxima

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11
Q

in interference, when there is phase difference of 2π, what is the path difference

A

1 wavelength

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12
Q

What is observed in Young’s Slits Experiment

A

Light from a single source directed to a double slit arrangement
Where the light from these slits overlaps, interference occurs and a pattern of bright and dark fringes is observed

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13
Q

In young slits experiment, why are the two light sources derived from a single laser

A

To ensure the two interfering sources of light are coherent

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14
Q

In Young’s slits experiment, how do we ensure that maximum diffraction occurs at the slits

A

Make the size of the slit in the same order of magnitude as the wavelength of the wave
Light is 10^-7 m

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15
Q

In Young’s slits experiment, explain why there is a bright fringe at the centre of the pattern

A

The light waves are in phase with each other as they leave the source
The waves travel the same distance so constructive interference occurs and a bright fringe results

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16
Q

In the equation
λ = ax / D
What does it all mean

A

In Young’s double slit experiment

Wavelength = ( separation of the slits X fringe separation) / slit to screen distance

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17
Q

In Young’s double slit experiment, what happens to the fringe spacing as
A) wavelength increases
B) separation between slits increases
C) distance between slit and screen increases

A

A) increases
B) decreases
C) increases

18
Q

How does the fringe pattern change as you cover the first slit

A

And interference pattern is formed

19
Q

How does the fringe pattern change if there was red light at the first slit and blue light at the second slit

A

The frequencies aren’t the same and therefore the sources aren’t coherent as wavelength is different
Therefore no interference pattern occurs

20
Q

What is the equation to find the critical angle from one substance to the next

A

sinC = n2 / n1

N2 is the less dense substance
N1 is the more dense substance

21
Q

what is diffraction

A

when waves spread around obstacles or through gaps

22
Q

what do you observe when a light from a laser is directed to a single slit of suitable width

A

a bright central band that extended well into the shadow region of the slit - central maximum
the bright band is bordered by regions of darkness - minima
moving out past the dark regions, the bright regions reduced in intensity - subsidiary maxima

23
Q

describe interference with sound waves

A

there is a loud sound at the centre of the interference pattern because the waves are in phase with each other
they travel exactly the same distance to the point at the pattern’s centre and so are still in phase with each other
constructive interference occurs so a loud sound is detected

24
Q

explain the meaning of interference

A

when 2 (or more) waves meet and there is a change in overall displacement

25
Q

what does the equation n=c/v mean

A

refractive index of the material = speed of light / speed of light in the material

26
Q

what’s the refractive index of air

A

1

27
Q

what’s snell’s law

apply it to nsin(θ) = constant

A

the angle that incoming light makes to the normal is called the angle of incidence
the angle the refracted ray makes with the normal is the angle of refraction

so
n1sin(θ1) = n2sin(θ2)

28
Q

what vital evidence did Young’s double slit experiment provide about diffraction and interference?

A

light could both diffract (through two narrow slits) and interfere ( to form the interference pattern on the screen)

29
Q

define path difference

A

the amount by which the path travelled by one wave is longer than the path travelled by the other wave

30
Q

Two coherent sources of sound waves each with a wavelength of 1.5m are set up so that they produce interference fringes. When the path difference is 3.75m, would you expect constructive or destructive interference?

A

find out how many times the wavelength goes into the path difference:
3.75 / 1.5 = 2.5

so the path difference is 2.5 X λ so the interference is destructive

31
Q

how can you demonstrate the diffraction of light

A

shining a monochromatic, coherent light through a very narrow slit onto a screen

32
Q

define monochromatic light

A

light of a single wavelength (so a single colour)

33
Q

if you narrow the slit, what happens to the diffraction pattern

A

becomes wider

34
Q

define diffraction grating

A

lots of equally spaced slits very close together

35
Q

what happens when a monochromatic light passes through a diffraction grating? why?

A

the interference pattern is really sharp because there are so many beams reinforcing the pattern

36
Q

in a diffraction grating pattern, what is the line of maximum brightness called
what are the lines called next to it

A

zero order line

the lines either side are called first order lines, then second order lines and so on

37
Q

how can you calculate the wavelength of light (using a diffraction grating)

A
  • pass the light through the grating
  • use the zero order and first order lines
  • measure the distance between the lines and the distance from the grating to the screen
  • work out the angle between them by using tan^-1(x / D)
  • work out the spacing between the slits in the diffraction grating
  • use the equation dsin(angle) = n X wavelength (n=1 and d is the slit spacing)
38
Q

in diffraction gratings what does it mean if:

a) the wavelength increases
b) split spacing increases

A

a) wavelength increasing means the angle gets bigger so the pattern will spread out
b) d increasing means the angle is smaller so the pattern comes closer together

39
Q

what happens when you diffract white light

A

zero order maximum is white

all the other orders form spectrums with red on the outside and violet on the inside

40
Q

what other factor do coherent waves have to have for interference to be noticeable during an experiment

A

the waves have to have approximately the same amplitude