Section 3 - Waves

Question 1

What is a progressive wave?

Answer 1

They carry energy from one place to another without transferring any material.

Question 2

What is a wave

Answer 2

The oscillation of particles of fields - caused by something making particles or fields oscillate (or vibrate) at a source, these oscillations pass through the medium (or field) as the wave travels, carrying energy with it

Question 3

How can you tell waves carry energy?

Answer 3

1. Electromagnetic waves cause things to heat up
2. X-rays and gamma rays knock electrons out of their orbits, causing ionisation
3. Loud sounds cause large oscillations of air particles which can make things vibrate
4. Wave power can be used to generate electricity
   Since waves carry away energy, the source of the wave loses energy

Question 4

What are the parts of a wave?

Answer 4

1. Cycle - one complete vibration of the wave
2. Displacement - how far a point on the wave has moved from its undisturbed position
3. Amplitude - maximum magnitude of displacement
4. Wavelength - the length of one whole wave cycle, from crest to crest, or trough to trough
5. Period - the time taken for a whole cycle (vibration) to complete, or to pass a given point
6. Frequency - the number of cycles (vibrations) per second passing a given point
7. Phase - a measurement of the position of a certain point along the wave cycle
8. Phase difference - the amount one wave lags behind another

Question 5

What is reflection?

Answer 5

When the wave is bounced back when it hits a boundary

Question 6

What is refraction?

Answer 6

The wave changes direction as it enters a different medium - the change in direction is a result of the wave slowing down or speeding up

Question 7

How are frequency and time period related?

Answer 7

Frequency is the inverse of the period 1Hz=1s^-1

Question 8

What are the ways you can draw a transverse wave?

Answer 8

1. They can be shown as graphs of displacement against distance along the path of the wave
2. They can be shown as graphs of displacement against time for a point as the wave passes
   Both graphs give the same shape, so check the axis

Question 9

What are transverse waves?

Answer 9

The oscillations of the particles are perpendicular to the propagation of energy

Question 10

What is a longitudinal wave?

Answer 10

Consists of alternate compressions and rarefactions of the medium it’s travelling through - when plotted graphically as displacement against time, they look like a transverse wave

Question 11

What is a polarised wave?

Answer 11

A transverse wave which only oscillates in one direction

Question 12

What is polarisation evidence for?

Answer 12

1808 - discovered that light was polarised by reflection, it was thought that light spread like sound, as a longitudinal wave, so it was hard to explain
1817 - Young suggested light was a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy - explained why light could be polarised

Question 13

How can you polarise light?

Answer 13

1. Ordinary light waves are a mixture of different directions of vibration which can then be polarised with polarising filters
2. If you have 2 polarising filters at right angles to each other then no light will get through
3. Light becomes partially polarised when reflected from some surfaces - some of it vibrates in the same direction

Question 14

What are the uses of polarised waves?

Answer 14

1. Polarised sunglasses - if you view reflected partially polarised light through a polarising filter at the correct angle, you can block out unwanted glare
2. Television and radio signals - The rods are all horizontal because the signals are polarised by the orientation of the rods on the broadcasting aerial and to receive a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting aerial

Question 15

What is the principle of superposition?

Answer 15

When 2 or more waves cross, the resultant displacement equals the vector sum of the individual displacements

Question 16

When is interference constructive or destructive?

Answer 16

Constructive - a crest and a crest or a trough and a trough
Destructive - a crest and a trough of equal amplitude cancel out
If the crest and the trough don’t have the same amplitude, then the destructive interference isn’t total

Question 17

How do you know the phase of a wave?

Answer 17

1. 2 points are in phase if they are both at the same point in the wave cycle - same displacement and velocity
2. 2 points with a phase difference of 0 or multiple of 360° (full cycle) are in phase
3. Points with a phase difference of odd numbers of multiples of 180° are exactly out of phase
4. 2 waves are usually in phase because they came from the same oscillator

Question 18

When are sources coherent?

Answer 18

If they have the same wavelength and frequency and a fixed phase difference between them, they need to be coherent (and in phase) to get a clear interference pattern

Question 19

What is path difference?

Answer 19

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

Question 20

When do you get constructive or destructive interference?

Answer 20

Depends on how much further one wave has travelled than the other wave to get to that point
Constructive - at any point an equal distance from 2 sources that are coherent and in phase or where the path difference is a whole number of of wavelengths - at these points the waves are in phase and reinforce each other
Destructive - where the path difference is half a wavelength, the waves arrive out of phase

Question 21

What is a stationary wave?

Answer 21

The superposition of 2 progressive waves with the same frequency (wavelength) moving in opposite directions - no energy is transferred

Question 22

How can you demonstrate a stationary wave?

Answer 22

By setting up a driving oscillator at one end of a stretched string with the other end fixed - the wave generated by the oscillator is reflected back and forth

Question 23

What are the different harmonics obtained by stationary waves?

Answer 23

First harmonic - stationary wave is vibrating at the lowest possible resonant frequency, it has one loop with a node at either end
Second harmonic - twice the frequency of the first harmonic, there are 2 loops with 3 nodes
Third harmonic - three times the frequency of the first harmonics, 1 and a half wavelengths

Question 24

How can powder be used to show stationary waves in a tube of air?

Answer 24

The lycopodium powder is laid along the bottom of the tube, and is shaken away from the antinodes but left undisturbed at the nodes

Question 25

How to investigate factors affecting the resonant frequencies of a string

Answer 25

1. Measure the mas and length of strings of different types using a mass balance and a ruler, then find the mass per unit length of the string µ
2. Set up the apparatus with one of the strings, and work out the tension using T=mg
3. Turn on the signal generator and vary the frequency
   Length - keep the mass per unit length and tension the same by moving the vibration transducer towards or away from the pulley, find the first harmonic again and and record f against l
   Tension - add or remove masses, keeping the mass per unit length the same, find the first harmonic again and record f against T
   Mass per unit length - use different string samples, keeping the length and tension the same, find the first harmonic and record f against µ

Question 26

What should the result from the factors affecting resonant frequencies be?

Answer 26

1. The longer the string, the lower the resonant frequency - half the wavelength at the resonant frequency is lower
2. The heavier the string, the lower the resonant frequency - waves travel more slowly down the string
   3.The looser the string the lower the resonant frequency - waves travel slower

Question 27

What is diffraction?

Answer 27

The spreading out of waves as they pass through an aperture or around objects

Question 28

What does the amount of diffraction depend on?

Answer 28

The wavelength of the wave compared to the aperture
1. When the aperture is a lot bigger than the wavelength, diffraction is unnoticeable
2. Diffraction is noticeable through a gap several wavelengths wide
3. You get the most diffraction when the gap is the same size as the wavelength
4. If the gap is smaller than the wavelength, the waves are mostly just reflected back

Question 29

What must be used to observe a clear diffraction pattern?

Answer 29

Use a monochromatic, coherent light source - lasers

Question 30

How to demonstrate light diffraction patterns with a laser

Answer 30

1. If the wavelength of light is about the same size as the aperture, you get a diffraction pattern
2. You will observe a central bright fringe (central maximum) with dark and bright fringes alternating on either side - caused by destructive and constructive interference

Question 31

What happens when white light is diffracted?

Answer 31

1. A mixture of different colours, each with different wavelengths
2. When white light is shone through a single narrow slit, all the different wavelengths are diffracted by different amounts
3. Instead of clear fringes, you get a spectra of colour

Question 32

Why is the central maximum the brightest part of the diffraction pattern?

Answer 32

1. The intensity of light is highest in the centre
2. Intensity is the power per unit area
3. For monochromatic light, all photons have the same energy, so an increases in the intensity means an increases in the number of photons per second
4. There are more photons per unit area hitting the central maximum per second than other bright fringes

Question 33

What affects the width of the central maximum in diffraction patterns?

Answer 33

1. Increasing the slit width decreases the amount of diffraction - this means the central maximum is narrower, and the intensity of the central maximum is higher
2. Increasing the wavelength increases the the amount of diffraction - this means the central maximum is wider, and the intensity of the central maximum is lower

Question 34

What is Young’s Double-Slit experiment?

Answer 34

1. To see two-source interference with light, you can either use two separate, coherent light sources or you can shine a laser (monochromatic and coherent) through two slits
2. Young’s Double-Slit experiment shines a laser through two slits onto a screen
3. The slits have to be about the same size as the wavelength of the laser light so that it is diffracted - then the light from the slits act like two coherent point sources
4. You get a pattern of light and dark fringes depending on whether constructive or destructive interference is taking place

Question 35

What are the laser safety rules?

Answer 35

1. Never shine the laser towards a person
2. Wear safety goggles
3. Avoid shining the laser beam at a reflective surface
4. Have a warning sign on display
5. Turn off the laser when it’s not needed

Question 36

How to do Young’s Double-slit with microwaves

Answer 36

Replace the laser and slits with two microwave transmitter cones attached to the same signal generator, and the screen with a microwave receiver probe
If you move the probe in a straight line, you’ll get an alternating pattern of strong and weak signals

Question 37

What is fringe spacing?

Answer 37

The distance from the centre of one minimum to the centre of the next minimum or from the centre of one maximum to the centre of the next maximum - very hard to get an accurate value of w (fringe spacing), it’s easier to measure across several and then divide by the number of fringe widths between them

Question 38

What is Young’s Double-Slit evidence for?

Answer 38

The nature of waves of EM radiation
1. Newton’s theory suggested light was made up of tiny particles called ‘corpuscles’ which could explain reflection and refraction
2. Huygens put forward a theory using waves that could explain diffraction and interference
3. Young’s Double-Slit provided the evidence that light could diffract through the narrow slits and interfere to form the interference pattern on the screne

Question 39

How can you make interference patterns stronger?

Answer 39

You can repeat Young’s Double-Slit experiment with more than 2 equally spaced slits with the same pattern but the bright bands are narrower and brighter, and the dark areas between are darker
Sharper fringes mean that measurements are more accurate

Question 40

What does monochromatic light on diffraction gratings look like?

Answer 40

1. All the maxima are sharp lines
2. There’s a line of maximum brightness at the centre called the zero order line
3. The lines either side of the central line are called first order lines, then second order etc

Question 41

How to derive the equation dsinθ = nλ

Answer 41

Watch a youtube video or something because I can’t turn that into a flashcard

Question 42

What conclusions can you draw from dsinθ = nλ?

Answer 42

1. If λ is bigger, sinθ is bigger, so θ is bigger- this means the larger the wavelength, the more the pattern will spread out
2. If d is bigger, sinθ is smaller - this means the coarser the grating, the less the pattern will spread out
3. Values of sinθ greater than 1 are impossible so if for a certain n you get a result of more than 1 more sinθ you know that order doesn’t exist

Question 43

How to use diffraction gratings to identify elements and calculate atomic spacing

Answer 43

1. If you diffract white light through a grating then the patterns due to different wavelengths are spread out by different amounts
2. Each order in the pattern becomes a spectrum, with red on the outside and violet on the inside - the zero order maximum stays white because all the wavelengths just pass straight through
3. Astronomers and chemists study spectra to help identify elements - they use diffraction gratings rather than prisms as they’re more accurate
4. The wavelengths of x-rays is a similar scale to the spacing between atoms in crystalline solids - x-rays will form a diffraction grating when directed at a thin crystal
5. The crystal acts like a diffraction grating and the spacing between atoms (slit width) can be found from the diffraction pattern
6. This is called x-ray crystallography - used to discover the structure of DNA

Question 44

What is the absolute refractive index?

Answer 44

A measure of optical density, it is the ratio between the speed of light in a vacuum and the speed of light in that material - property of that material only

Question 45

What is relative refractive index?

Answer 45

The ratio of the speed of light in that material - property of the interface between 2 materials

Question 46

How does the optical density affect the light as it enters a material?

Answer 46

Light goes fast in a vacuum, it slows down in other materials because it interacts with the particles in them, the more optically dense a material is, the more light slow downs when it enters it

Question 47

What is the refractive index of air?

Answer 47

1
This means that you can assume the refractive index for an air to glass boundary equals the absolute refractive index of the glass

Question 48

What is Snell’s law?

Answer 48

n1sinθ1 = n2sinθ2
1. The angle the incoming light makes to the normal is called the angle of incidence θ1
2. The angle the refracted ray makes with the normal is the angle of refraction θ2
3. The light passes from a material with a refractive index of n1 to a material with a refractive index of n2
4/ When light enters a optically denser medium, it is refracted towards the normal

Question 49

What happens when light leaves an optically denser material?

Answer 49

The light is refracted away from the normal

Question 50

What happens as you change the angle of incidence?

Answer 50

1. The light is refracted away from the normal until the critical angle
2. Eventually θ1 reaches the critical angle, and the light is refracted along the boundary
3. When θ1 is greater than the critical angle, refraction is impossible and all the light is refracted back to the material - Total Internal Reflection
   Since sin can only take values between -1 and 1, TIR can only happen if sinθc is less than 1 so 1n2 is less than 1

Question 51

What are optical fibres?

Answer 51

A very thin piece of glass or plastic fibre that can carry light signals over long distances and around corners

Question 52

How do optical fibres work?

Answer 52

1. Step-index optical fibres themselves have a high refractive index but are surrounded by cladding with a lower refractive index to allow TIR - cladding also protects the fibre from scratches which could let light escape
2. Light is shone at one end of the fibre - the fibre is so narrow that the light hits the boundary between the fibre and cladding at an angle bigger than the critical angle
3. All the light is TIR from boundary to boundary to boundary until it reaches the other end

Question 53

What does signal degradation do?

Answer 53

Causes information to be lost
A signal (a stream of pulses of light) travelling down an optical fibre can be degraded by absorption or by dispersion.

Question 54

What is absorption in signal degradation?

Answer 54

As the signal travels, some of its energy is lost through absorption by the material the fibre is made from, this energy loss results in the amplitude of the signal being reduced.

Question 55

What is modal dispersion in signal degradation?

Answer 55

Light rays enter the fibre at different angles, and so take different paths. The rays which take a longer path take longer to reach the other end than those that travel down the middle of the fibre. A single-mode fibre is used to stop it as it only lets light take one path

Question 56

What is material dispersion?

Answer 56

Light consists of different wavelengths that travel at different speeds in the fibre - this causes some light wavelengths to reach the end of the fibre faster than others. Monochromatic light can stop it, as there is only one wavelength

Question 57

What is dispersion in signal degradation?

Answer 57

Causes pulse broadening.
The signal sent down the fibre is broader at the end, broadened pulses can overlap each other and confuse the signal. An optical fibre repeater can be used to boost and regenerate the signal every so often which can reduce signal degradation caused by both absorption and dispersion