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Physics > Module 4 Waves > Flashcards

Module 4 Waves Flashcards

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1
Q

Define transverse and longitudinal waves.

A

Transverse waves have particles vibrating perpendicularly to the direction of energy transfer.

Longitudinal waves have particles vibrating parallel to the direction of energy transfer.

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2
Q

What are progressive waves?

A

Waves that transfer energy away from a source.

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3
Q

Define wavelength and displacement.

A

Wavelength of a wave is the distance between two successive identical points on a wave.

Displacement is the distance moved by a particle from it’s rest position, in a wave.

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4
Q

Define frequency and time period. What equation combines them both?

A

Frequency of a wave is the number of vibrations of any point in a wave per second.

Time period is the time taken for one wave cycle(one complete pattern of oscillation)to complete.

F=1/Th

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5
Q

Define phase difference and state it’s units.

A

Phase difference is the difference in pattern of vibration between two points.
It is usually measured in pi radians but can also be measured in degrees or even differences in wave cycles.

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6
Q

What does a displacement against time graph show? What properties can you derive about the wave?

A

The graph shows the pattern of vibration of individual particles in a wave.
We can derive the time period, frequency, displacement and amplitude.

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7
Q

What does a displacement against distance graph show? What properties can you derive about the wave?

A

The graph shows the pattern of vibration of a wave.

We can derive the wavelength, amplitude and displacement.

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8
Q

What does it mean for two particles to be in phase and to be in anti-phase?

A

Two particles with the same pattern of vibration are in phase.
Particles with exactly the opposite pattern of vibration are said to be in anti-phase.

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9
Q

How many radians are two points out of phase if they are half an oscillation cycle different?

A

1 Pi radian.

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10
Q

What waves can be polarised and why?

A

Only transverse waves can be polarised as they have particles vibrating perpendicularly to the direction of motion. This causes the unpolarised waves to oscillate in many planes and hence allows the polariser to block out all unwanted waves from going through.
(Whereas in a longitudinal wave all vibration is along the horizontal direction)

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11
Q

How can you identify rarefactions and compressions from displacement-distance graph for longitudinal waves?

A

Both events are at points where displacement is zero. However the point of compression has an area of positive displacement before it and an area of negative displacement after it. It is the opposite for rarefactions.

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12
Q

Where are the compressions and rarefactions on pressure-distance graphs?
What is the phase difference between pressure-distance and displacement-distance graphs p, in pi radians for longitudinal waves?

A

Compressions are signified by positive amplitude and rarefactions are signified by negative amplitudes.
The phase difference is 90 degrees and hence 1/2 pi radian

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13
Q

What is the “time-base” of an oscilloscope and how can you use it to find the frequency of a wave?

A

The time base tells us the time taken for the dot to travel 1 horizontal division on an oscilloscope.

Make sure to convert the value of time base into seconds and multiply this value by the no. of divisions one complete wave cycle is represented in. This is the time period ‘T’.
F=1/T

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14
Q

State the 2 formulae for the speed of a wave.

A

v=f x lambda

v=lambda/T

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15
Q

Define intensity. State it’s units.

A

Intensity is the rate of energy transfer per unit area. (Power transfer per unit area).
It’s units are W/m^2 or J/m^2s

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16
Q

If a speaker outputs sound across a room of length 1m, what is the area covered by the sound waves?

A

We can assume the waves flow uniformly in all directions.
Hence we can use: 4Pir^2.
4xPix1^2
12.6m2

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17
Q

What is the relation between intensity and amplitude?

A

Intensity is directly proportional to the square of Amplitude.

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18
Q

What are Wavefronts?

A

Lines drawn to represent points in phase in a wave, usually crests.

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19
Q

What happens to the wavelength and frequency of a wave after it undergoes refraction?
What happens to the wave as a result of these changes?

A

Wavelength changes but frequency remains the same.

The speed of the wave changes.

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20
Q

Describe an experiment to investigate reflection and refraction.

A

A student can take a ripple tank, pour some liquid on the surface and create waves in the liquid using a dipper. They can then examine the waves by shining a light on top of the liquid and observing the shadow wave pattern on a screen below.
For reflection they can put certain objects in front of the waves and for refraction they can introduce a shallower region on the tank

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21
Q

Define diffraction.

A

It is the spreading out of waves after passing through a gap.

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22
Q

How do you ensure maximum diffraction?

A

Make the gap equal to the wavelength.

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23
Q

Define interference.

A

Interference is the addition of two or more waves that produces a new wave pattern.

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24
Q

Describe the components of an electromagnetic wave.

A

E.m waves have a magnetic field and an electrical field oscillating in phase together but at 90 degrees to each other and direction of travel.

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25
Q

What is the range of wavelength for visible light?

A

740nm to 370nm

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26
Q

What are the origins of x rays and gamma rays within an atom?

A

X rays are released due to the accelerated electrons outside the nucleus, whereas gamma rays are released from within the unstable nucleus of an atom.

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27
Q

Give the names along with orders of magnitude of the groups in the electromagnetic spectrum.

A 
Radio: 10^4 - 10^-1
Microwaves: 10^-1 - 10^-4
Infrared: 10^-3 - 7.4x10^-7
Visible light: 7.4x10^-7 - 3.7x10^-7
U.V: 3.7x10^-7 - 10^-9
X-rays: 10^-7 - 10^-12
Gamma: 10^-9 - 10^-16
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28
Q

What are the 6 properties of EM waves?

A

They travel at the speed of light.
They can travel in a vacuum.
They can be polarised.
They can be reflected, refracted and diffracted.
They have an electric wave and a magnetic wave in phase but oscillating at 90 degrees to each other.
They are all transverse waves.

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29
Q

What part of the EM spectrum has waves that are not ionising?
What is needed for a wave to become ionising radiation?

A

Radio waves, Microwaves, Infrared and visible light waves are non ionising.
Ionising radiation requires a certain amount of photon energy to be able to knock off electrons from an atom.

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30
Q

What three wave groups is UV radiation from the Sun divided in?
What is the fate of each type?

A

UV A; causes tanning (about 99% of sunlight)
UV B; causes sunburn and cancer
UV C; is reflected by the Earth’s atmosphere.

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31
Q

What radiation does sunscreen block?

A

Blocks UV B

32
Q

Describe the appartus of an X-ray machine.

A

It has a high voltage source of emf providing current to a filament. The filament is in an evacuated tube, the tube also has a copper anode fitted in it. The filament fires high energy electrons at the copper anode, which releases X-rays. The copper anode is connected back to the high voltage source.

33
Q

What is a plane polarised wave?

A

A wave that only oscillates in one plane.

34
Q

Define a method to observe polarisation in Light.

A

Using two polaroids, we can first polarise unpolarised light sources using the first polaroid, named the polariser. Using the second polaroid called the analyser we can view the effects of polarisation on plane polarised waves by rotating the analyser in relation to the polariser.

35
Q

In what position does a metal grill have to be in, relative to the plane of a plane polarised microwave, to allow complete transfer and no transfer?

A

Perpendicular for complete transfer and parallel for no transfer.

36
Q

What does an analyser do the incoming plane polarised wave?

A

It polarises it into another plane, however the amplitude of the wave is reduced and the intensity is reduced exponentially.

37
Q

In what case will a wave not change direction while undergoing refraction?

A

If the angle of incidence is 90 degrees, ie the wave is along the normal.

38
Q

When will the speed of a wave increase while refracting?

A

If it moves from a material with a higher refractive index to a material with a lower refractive index.

39
Q

If a wave slows down while refracting, will it move closer or further away from the normal, while exiting the original medium?

A

Closer to the normal.

40
Q

Is the angle of refraction bigger or smaller than the angle of incidence, if a wave speeds up?

A

The angle of refraction is bigger.

41
Q

Describe an experiment that calculates the refractive index of a transparent rectangular material.

A

Outline the material on a sheet using a marker, draw a line perpendicular to the surface of one of the edges, this is the “normal line”. Using a light box shine the light at angles such that the point of incidence is where the normal meets the surface of the material. Trace the incident light and outgoing light and connect the two lines by a straight line. Measure the angles of incidence and refraction for three different positions of the light box and then either divide the sin of avg of angles of incidence by the avg of sin of angles of refraction or plot a graph of sin angle of incidence against sin angle of refraction. The gradient will be the refractive index.

42
Q

What is the formulae used to calculate refractive index of a material, using the speed of light in a material?

A

n=c/v

43
Q

What two equations are for Snell’s Law?

A

n1 x sin Angle of Incidence =n2 x sin Angle of Refraction

n x sin Angle to the normal = k

44
Q

What is a unique property of a semi circular block related to refraction.

A

The light exiting from within the semi circular block through the curved area does not change it’s direction, as the angle of incidence is 90 degrees.

45
Q

Is reflection taking place while refraction occurs?

A

Yes but only a small amount of waves are reflected.

46
Q

What 2 things are required for total internal reflection?

A

The incident ray to be travelling from a material with a higher refractive index to a material with a lower refractive index.
The angle of incidence to be higher than the critical angle.

47
Q

What happens if the incident light is at critical angle?

A

The refracted ray will travel along the surface of the second medium and have angle of refraction of 90 degrees.

48
Q

Derive the equation for critical angle if the medium with the lower refractive index is air.

A 
n1 x sin Theta 1= n2 x sin Theta 2
Theta 1 = Angle of incidence = Critical Angle = C
n1 x sin C=n2 x sin Theta 2
n2 in air is 1
n1 x sin C=1 x Theta 2
Theta 2= 90 degrees when Theta 1 = C
n1 x sin C=1 x1
sin C= 1/n1
49
Q

State the equation for critical angle in cases where n2 is not for air.

A

sin C=n2/n1

50
Q

How can you figure out critical angle of a semi circular material and hence the refractive index, by experiments?

A

Trace the edges of the material on a sheet of paper, draw a normal line perpendicular to the centre of the straight line part of the semi circular block.
Shine a light, using a ray box, through the curved area such the point of incidence is at where the normal meets the surface of the block. Adjust the ray box till the no light is refracted back into the block and it is travelling along the surface. Measure the angle of incidence here, this will be the critical angle.

Using the equation sin C=n2/n1, calculate value for n1; n1=n2/sin C

51
Q

Is refraction taking place during total internal reflection?

A

Nope.

52
Q

State the principle of superposition.

A

When two or more waves of the same type meet, the resultant wave is the sum of the displacements of the individual waves

53
Q

When would destructive interference occur?

A

When waves of the same type and amplitude but 180 degrees out of phase interfere.

54
Q

Define what is meant by coherent waves.

A

Waves with a constant phase difference.

55
Q

Is light from a bulb coherent? Give an example of sources of coherent light.

A

Nope. Laser light is coherent.

56
Q

What is the path difference during destructive interference?

A

1/2 Lambda

57
Q

When will constructive interference occur,regarding path difference?

A

At any whole number multiple

58
Q

Describe two source interference of sound.

A

Upon superposition there will be areas of louder sound followed by areas of softer sound created. The distance between the two is increased by using lower frequency sounds.

59
Q

Define an example of two source interference for microwaves.

A

Within radar systems are tubes called waveguides. These are tubes that are split into two tubes from one tube and then the split tubes rejoin into one continious tube. This split allows microwaves to split apart and travel different distances through each split tube. The remainder of the distances travelled by individual microwaves, tells us about whether the final interference would be constructive or destructive.

60
Q

How does Young’s double slit experiment confirm light as a wave?

A

It shows interference and superposition, which confirms light as a wave.

61
Q

State formula for calculating wavelength by Youngs double slit.
For what circumstances does the formula apply?

A

Lambda = ax/D

This is only true when a is much less than D and the angle of separation from central fringe is less than 10 degrees.

62
Q

What is the fringe spacing? How can you reduce uncertainty while measuring it?

A

The distance between two maximas or minimas.

Measure the distance for 10 fringes and find the average

63
Q

Is wavelength calculated more accurately from using double slits or diffraction gratings?
Why is this so?

A

Diffraction gratings, as the increase in number of slits allows for a brighter and sharper maximas, compared to blurry maximas produced by double slits. The maximas are also further apart, reducing percentage uncertainty.

64
Q

Describe how you calculate wavelength using diffraction gratings.

A

Shining a coherent source of light in phase through a diffraction grating produces sharp and bright maximas on a screen. We can calculate wavelength by using the formulae nxlambda=dxsinTheta, where n is the no. of the order of maxima being investigated, d is the slit separation and theta is the angle between the zero order and n from the diffraction grating.

65
Q

Describe how you would calculate wavelength using double slits.

A

Shining a coherent light source through double slits diffracts the waves and through interference maximas and minimas are produced on a screen. We can calculate wavelength Using the formula Lambda=ax/D, where a is a is the split separation, x is the fringe distance and D is the distance from the double slits to the screen.

66
Q

How can you find the maximum number of maximas displayed by a diffraction grating for a particular wavelength of light?

A

By dividing the d by LAMBDA

67
Q

How are stationary waves formed?

A

When progressive waves travelling in one direction are reflected back towards the source, the superposition of these two waves of same frequency and roughly the same amplitude but opposing direction of travel produce stationary waves.

68
Q

Define nodes and antinodes.

A

Nodes are regions where destructive interference takes place and amplitude of resultant wave is zero.
Antinodes are regions where constructive interference occurs and amplitude of resultant wave is twice the individual amplitude of one of the progressive waves.

69
Q

What is the wavelength between two nodes?

Is this the same as the distance bw two antinodes?

A

1/2 a wavelength.

Yup.

70
Q

What is the first harmonic?

How many nodes will be present on the 7th harmonic?

A 
This is the lowest possible resonant frequency, which is required to form a standing wave.
8 nodes (number of nodes = no of harmonic + 1)
71
Q

Describe an experiment on standing waves using microwaves and Define how you can calculate the wavelength of the microwave.

A

A metal reflector is used to reflect incoming microwaves from a microwave transmitter. The superposition of the incoming and reflected waves produces standing waves at a certain frequency called the resonant frequency. By moving a receiver along the standing wave we can identify nodes and antinodes.
The distance between 10 nodes can be measured using a metre rule and the avg distance will equal avg distance bw consecutive nodes. This is equal to 1/2 a wavelength. So we can times the avg by 2 to figure out wavelength of microwave.

72
Q

In a closed pipe, what is the whole number multiple of the fundamental frequency at the 5th harmonic?

A

9 times the Fundamental frequency (as there are odd number increments in the number multiplied with the fundamental frequency, for every increment in number of harmonics)

73
Q

How is the fundamental frequency of an open tube related to the fundamental frequency of a closed tube?

A

2 times the fundamental frequency at the closed tube.

74
Q

Are all points on a standing wave in phase between 2 nodes?

A

Yes.

75
Q

What is x in Lambda=ax/D?

A

Fringe separation; distance bw two minimas or maximas

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