Waves and the particle nature of light Flashcards
(85 cards)
1
Q
What are the 3 descriptions of waves
A
- Stationary vs progressive
- Longitudinal vs transverse
- Mechanical vs electromagnetic
2
Q
What is a wavefront
A
- Particles in a wave along a particular crest that are all in phase
- always 90 degrees to direction of wave motion
3
Q
What is Huygen’s principle
A
- Every point on a wavefront is a secondary source of spherical wavelets that spread out with wave velocity
- The new wavefront is the envelope of the secondary wavelets
4
Q
What is the principle of superposition
A
- If 2 or more waves meet at a point, the resultant displacement is the vector sum of the displacements of the separate waves
5
Q
What is constructive interference
A
- 2 waves meeting in phase- amplifying each other
6
Q
What is destructive interference
A
- 2 waves meeting in anti phase- nullifying/canceling out each other
7
Q
When discussing path difference….
A
Always speak about phase difference as well (and vice versa)
8
Q
What is path and phase difference in constructive interference
A
- Whole number of wavelength
- 0 degree phase difference
9
Q
What is path and phase difference in destructive interference
A
- Odd number of half wavelengths
- 180 degree phase difference
10
Q
A wave is coherent when:
A
- Waves are same type
- Waves are same frequency (wavelength)
- Sources maintain a constant phase difference- if in phase, stay in phase etc
11
Q
When is there high amounts of diffraction
A
- When the gap is much smaller than the wavelength
12
Q
When are there low amounts of/no diffraction
A
- When the gap is much larger than the wavelength
13
Q
When does diffraction around an obstacle occur
A
- If the obstacle is of the same order of magnitude of the wave
14
Q
The greater the wavelength (diffraction)….
A
- The greater the diffraction around an obstacle
- Eg drums can be heard around the street corner before the high notes
15
Q
When are interference patterns observed
A
- When waves are coherent
- When sources have similar amplitude
16
Q
What are bright fringes in a diffraction grating
A
- Where light meets in phase producing constructive interference
17
Q
What are dark fringes in a diffraction grating
A
Where light meets in anti phase to produce constructive interference
18
Q
What is d in diffraction grating equation
A
- distance between slits (m)
19
Q
What is n in diffraction grating equation
A
- Order of fringe
20
Q
What is θ in diffraction grating equation
A
- angle of diffraction
21
Q
Why is a double slit used for young’s experiment
A
- The first screen is has a single slit to ensure coherence
- Second screen with 2 slits used to create interference fringe pattern
22
Q
Why can a laser replace the original light source and the first slit
A
- Laser is a coherent source
23
Q
Which colour in the spectrum of light will diffract the most
A
- Red as it has the largest wavelength- red fringes further apart than violet fringes
24
Q
What happens so fringe separation and brightness if slit separation is decreased
A
- Separation increases
- Brightness unaffected
25
What happens to fringe separation and brightness if D (distance from screen) increases
- Separation increases
- Brightness decreases
26
What happens so fringe separation and brightness if wavelength of light increases
- Separation increases
- Brightness unaffected
27
What happens to fringe separation and brightness if one of the double slits is covered
- Single slit diffraction pattern
- Brightness decrease
28
What happens to the fringes if white light is used
- Central white fringes, with spectral fringes either side- red furthest from centre
29
What happens to fringe separation and brightness if source slit is widened
- Separation decreases
- Brightness increases
30
What happens to fringe separation and brightness if single slit is moved closer to double slits
- Separation decreases
- Brightness increases
31
Properties of a progressive wave
- Transfers energy in direction of wave travel
- All points along the wave have same amplitude
- Adjacent points in the wave have different phase relationship- only in phase 1 wavelength apart
32
What is a stationary wave
- Produces when 2 identical waves travelling in opposite directions meet and interfere
33
properties of stationary waves
- Energy stored within each vibrating particle
- Amplitude varies between a max value at antidote and min value at node
- All points between each pair of consecutive nodes have constant phase relationship
34
What type of waves can be plane polarised
- Transverse due to direction of oscillation
- Electromagnetic
35
Which direction do unpolarised waves oscillate in
- All directions
36
What happens when light travels through polarisation filter
- Oscillations in all directions bar one will be absorbed
- Emerging light will be plane polarised
37
What is intensity also known as
- Radiation flux
38
Properties of mechanical waves
- Require a medium for transmission
- Generated by vibrating sources
- Can be longitudinal or transverse
- Energy from vibration is transmitted through the medium but medium itself remains undisturbed once energy passed through
39
Properties of em waves
- Don't need a medium, can travel through a vacuum
- Travel at same speed in vacuum (C)
- All transverse waves
- Consist of oscillating electric and magnetic fields that are in phase
40
Wavelength range of visible light
- 400-700 nm
41
Wavelength range of uv waves
- 10^-8 to 10^-10 m
42
Wavelength range of ir waves
- 10^-3 to 10^-7 m
43
Human hearing range
- 20-20,000 Hz
44
What is acoustic impedance
- how much reflection of sound off a medium occurs
- depends on density
45
What is attenuation
- Energy absorbed or scatter
- Not reflected back to receiver
46
Why does ultrasound involve sending wave pulses
- To allow sufficient time for the signal to reach the target of interest and be reflected back to the transducer before the next pulse is generated
47
How can resolution of ultrasound be increased
- Increasing the frequency
- However smaller wavelength is more easily absorbed (More attenuation)
48
What is refraction
- 'Bending' of light when travelling from one medium to another
49
Why does refraction occur
- Change in speed as light enters a new medium results in change in wavelength
50
What is refractive index of air
- 1- smaller than all other materials
- This because n=c/v, so it will always be larger than 1
51
What happens to light waves when they enter a medium with high n
- Slows down
- therefore moves towards the normal
52
What happens to sound waves when enters medium with larger density
- Speeds up as particles closer together
53
When does total internal reflection occur
- When light is travelling from a high to low refractive index
- Angle of incidence > Critical angle
54
What is the critical angle
- The largest angle of incidence for which refraction can occur
- The refracted wave is travelling at 90 degrees to normal
55
What is a converging lens
- Lens that brings parallel rays to a REAL focus- light can be focussed on a screen
56
If a lens is strong...
- The rays bend more- shorter focal length and vice versa
57
What is a diverging lens
- Lens that brings parallel rays to a virtual focal point
58
How is an image created by rays
- At least 2 rays
- One ray from the top of object parallel to the principle axis and refracted through it
- One ray from top of the object through the centre of the lens
59
What is U (lens)
- Distance from object to lens
60
What is V (lens)
- Distance from image to lens
61
What is f (lens)
- Focal length
62
What is a real image
- An image that can be displayed own a screen, as the light actually passes through a point
63
What is a virtual image
- An image that cannot be displayed on a screen, as the light doesn't actually pass through a point, it just appears to
64
What is f for a converging lens
- Positive
65
What is f for a diverging lens
- Negative
66
What is ground state
- The lowest energy level, where the atom is most stable
67
What is excited state
- Any higher energy level (than ground) reached by absorption of energy in the electron
68
What happens to an electron in excited state
- Becomes unstable and therefore falls back to ground state, releasing energy in the form of em radiation
69
Energy of a photon
- E=hf
- h=planck's constant
- f=frequency
70
What happens when an electron reaches ionisation level
- Has enough energy to break bond and become free electron
- If has any left over energy it will be used as KE
71
How is threshold frequency evidence for particle model of light
- Energy of a photon = hf
- Emission only occurs if frequency above value
72
How is Instantaneous emission of electrons evidence for particle model of light
- If light was a wave, the even distribution of wavefronts would take time for an electron to gain enough energy to escape- so energy must be discrete- photons
73
How is electrons emitted being proportional to intensity evidence for particle model of light
- One photon is absorbed by 1 electron so an increased number of photons (intensity increase) leads to increased number of electrons emitted
- Means energy is discrete not continuous (Like waves)
74
How is increasing frequency leading to increased KEmax of electrons evidence for particle model of light
- Energy of photon= hf
- Increasing frequency will increase energy in the photon, therefore creating excess energy for the electron after escaping to be used as KE
- hf= min energy needed to escape + KEmax
75
How is intensity having no effect on KE of electrons evidence for particle model of light
- Intensity is linked to the number of photons, so increase the number of photons doesn't increase energy per photon
76
What is work function
- The minimum energy needed by an electron to escape from the surface
77
What is KEmax at threshold frequency
- 0
- Therefore hf = work function
78
What are the observations of photoemission
1- Emission only occurs if incident radiation is above Threshold frequency
2- Emission occurs instantaneously when radiation starts
3- Number of electrons emitted is proportional to intensity of radiation (Not frequency)
4- Electrons emitted with a range of KEs from 0 to max- increasing frequency increases the KEmax of electrons
5- Intensity has no effect of KE of electrons
79
What did De Broglie observe
- When a beam of electrons passes through a very thin sheet of crystals of graphite or a very small circular hole, electrons are diffracted and a pattern of fuzzy concentric circles are observed
80
What is the de Broglie equation
Associated wavelength= h/p
81
What do large momentum lead to (de broglie)
- Smaller rings due to less diffraction
- because associated wavelength is smaller
82
What is De Broglie's observation an example of
- Particles acting like waves
83
How to convert J to eV
- DIVIDE by electron charge (1.6x10^-19)
- remember eV is smaller so value will be larger
84
How to convert eV to J
- MULTIPLY by electron charge (1.6x10^-19)
- remember J is larger so value will be smaller
85
