Waves - optics Flashcards
(67 cards)
1
Q
what does it mean if a wave is coherent?
A
Waves must have the same frequency and a constant phase difference
2
Q
how does 2 source interference occur?
A
if 2 sources of waves are coherent and superpose to form an interference pattern
3
Q
what is constructive interference?
A
where 2 waves in phase superpose to give a wave with a larger amplitude
path difference = whole no of wavelengths or even no of pi
4
Q
what is destructive interference?
A
when 2 waves out of phase superpose to cancel each other out
path difference = n+1/2 wavelengths or odd no of pi
5
Q
what is path difference
A
the difference in distance travelled by 2 waves to the point at which they superpose
6
Q
what is diffraction?
A
- it is when waves pass through an opening/slit or around an obstacle and they spread out to occupy areas that would otherwise be in shadow
- the longer the wavelength the greater the diffraction
- the smaller the aperture the greater the diffraction
7
Q
what happens during single slit diffraction?
A
- when light is incident on a narrow slit with a width comparable to its wavelength, it diffracts through, spreading out in different directions
- this allows the slit to act like a point source of light
8
Q
what is the interference pattern produced for monochromatic light during single slit interference?
A
there is a broad and bright central maximum, which is twice as wide as side maxima. intensity of maxima decreases as you get further away from the centre
9
Q
how does the interference pattern change for monochromatic light single slit diffraction as you increase the wavelength?
A
- increases the width of all maxima
- decreases the intensity of the central maxima
10
Q
how does the interference pattern change for monochromatic light single slit interference as you make the slit narrower?
A
the fringes become wider
11
Q
how do you find phase difference between 2 rays?
A
ΔΦ = 2π Δl/λ
12
Q
what diffraction pattern is produced when white light diffracts through a single slit?
A
- different wavelengths are diffracted by different amounts
- broad bright white central maximum with dimmer coloured fringes either side of it which are wider apart, with blue closest to the central maximum and red furthest
13
Q
what is the fringe separation equation based on Young’s double slits?
A
w = λD/s
where:
w = fringe separation
λ = wavelength
D = horizontal distance from the screen
s = slit separation
14
Q
what is fringe separation?
A
the distance between the centres of 2 adjacent maxima or minima (bright or dark fringes)
15
Q
how does wavelength of light affect fringe separation using a double slit?
A
red light has a larger slit separation than blue light because red light has a linger wavelength
16
Q
describe the fringe pattern produced for monochromatic light double slit diffraction?
A
evenly spaced light and dark fringes with equal intensity
17
Q
when slit separation is small…
A
…the rays can be approximated as parallel
18
Q
why does young’s double slit experiment show light is a wave?
A
the resulting fringe pattern must be due to diffraction and interference, which cannot happen with a particle
19
Q
describe the fringe pattern for double slit diffraction of white light?
A
There is a white central maximum and coloured side fringes with equal spacing and intensity, with blue closest to the centre and red furthest away
20
Q
how does two source interference work with sound/speakers?
A
- 2 speakers = coherent sources if driven from the same signal generator (same freq + constant phase diff)
- waves superpose - single slit diffraction patterns from speakers overlap
- when path diff = integer no of wavelengths, 2 waves are in phase, maxima observed - sound is loudest here
- when path diff = n+1/2 wavelengths, 2 waves are in antiphase, minima observed - no sound heard here
21
Q
describe young’s double slit experiment
A
- illuminate a double slit with the slits close enough together - either use a laser or other monochromatic coherent light, or use monochromatic light passed through a single slit before going through the double slit
- each closely spaced slit produces a single slit diffraction pattern
- these patterns overlap, forming the double slit diffraction pattern seen
22
Q
how are the bright and dark fringes produced by double slit diffraction explained?
A
- bright fringes: path diff between light from each slit = integer no of wavelengths - waves arrive in phase at the screen + superpose constructively
- dark fringes: path diff between light from each slit = n+1/2 wavelengths - waves arrive out of phase + superpose destructively
23
Q
what are vapour lamps and discharge tubes like as light sources?
A
- produce light with a dominant wavelength
- eg. sodium vapour lamp emits mainly 590nm lilght - other wavelengths also emitted but the dominant one is 590nm
- effectively a monochromatic light source since its spectrum is dominated by light of a certain wavelength
24
Q
what is the sun or filament bulbs like as light sources?
A
- composed of colours from all parts of the visible light spectrum - continuous range
- NOT a monochromatic light source
25
what are lasers like as a source of light?
it is a monochromatic and coherent light source
26
how can lasers be used safely in a laboratory?
- turn off when not in use
- never shine it at a person
- avoid shining at reflective surfaces
- display a warning sign on the lab door
- never look directly at it, even the reflected ray
- wear laser safety googles if using highly powered lasers
27
how do you find slit separation in a diffraction grating?
d = (1*10^-3)/(lines per mm)
28
what is a diffraction grating?
a plate with many closely spaced parallel slits ruled on it
29
why is light transmitted in certain directions only by a diffraction grating?
light passing through each slit is diffracted to produce a single slit diffraction pattern. diffracted light waves from the different slits interfere with each other (mostly destructively) and only superpose constructively in certain directions only. this causes light to be transmitted in certain directions only and cancelled out in all other directions.
30
what is the diffraction pattern produced when monochromatic light diffracts through a diffraction grating?
narrow maxima are produced that get further apart the further they get from the central maximum - maxima are much sharper than those produced by single and double slit diffraction
31
what is the diffraction pattern produced when white light diffracts through a diffraction grating?
there is a narrow white central maximum, all other orders of maxima are spectra, they start overlapping with each other beyond the 2nd order since the spectra are so broad.
32
how can you increase the angle of diffraction between the central maximum and all other maxima?
- use light of a longer wavelength
- use a grating with slit closer together (so more lines per mm)
33
what is the equation used to find the angle of diffraction between 2 maxima with a diffraction grating?
d sinθ = nλ
where:
d = slit seperation
n = order of maxima
λ = wavelength
34
what is the maximum possible angle of diffraction?
90 degrees
35
how can you calculate the maximum order of diffraction that can be observed using a diffraction grating?
n max = |d/λ|
(always round the answer down)
36
how are diffraction gratings used?
- astrophysics - analyse spectra of stars, galaxies, etc to determine composition, temperature, etc
- to measure the wavelength of a laser (if line spacing is known)
- chemical analysis + identification using absorption spectrum colorimetry
37
what are the issues with using a higher order maximum of a diffraction grating pattern?
while more lines are visible due to being more spaced out, they are also wider which makes it harder to identify the exact diffraction angle, leading to a reduction in accuracy
38
what is a continuous spectrum?
- eg. produced by a white filament bulb
- ranges from 350nm-650nm
- most intense part of the spectrum depends on the light source - the hotter the light source the shorter the wavelength of the brightest part
- by calculating this wavelength you can measure the temp of a light source
39
what is a line emission spectrum?
- glowing gas in a vapour lamp/discharge tube emits light of specific wavelengths - specific to the gas used
- this produces a spectrum of narrow vertical lines of different colours that correspond to the wavelengths of light emitted
- light emitted by a compound will have the emission spectra of the elements that make it up on top of each other - can be used to identify the elements making up a compound
40
what is an absorption spectrum?
- a continuous spectrum with dark lines along it at certain wavelengths
- basically the opposite of a emission spectrum
- the elements on a vapour lamp absorb light of the same elements they emit, causing the missing wavelengths to appear
- absorbed light is re-emitted but not in the same direction as the transmitted light
41
what is refraction?
it is the change of direction that happens when light passes a an angle across a boundary between 2 transparent substances
42
how fast does light travel in a vacuum (and how does this compare to other substances)?
in air: 3.00 * 10^8 m/s
it travels slower in all other mediums
43
how does a light ray bend when it moves between a glass block and air?
- air to glass - bends towards the normal
- glass to air - bends away from the normal
44
how does a light ray bend at the boundary between 2 substances?
- bends towards the normal if passing into a more dense substance
- bends away from the normal if passing into a less dense substance
45
what is the refractive index of a substance?
the ratio of the speed of light in a vacuum (c) to the speed of light in that substance (cs):
n = c/cs
46
what is the refractive index of air?
approximately 1
47
what is the relationship between wavelength and refractive index?
they are inversely proportional
n1λ1 = n2λ2
48
what is Snell's law in words?
when a light ray strikes the boundary between 2 media it is always partially refracted
49
what is the Snell's law equation?
n1sinθ2 = n2sinθ2
50
how do you measure angles of incidence, reflection, refraction etc.?
from the normal
51
what are the 3 rays of light present in a ray diagram of refraction?
- incident ray
- reflected ray (from partial reflection)
- refracted ray
52
how do the different colours in white light refract?
- BLUE BENDS BEST - blue/violet light is refracted more than red light
- this causes white light to separate out into a spectrum - dispersion
53
when does total internal reflection happen?
when light is travelling from a more optically dense material to a less optically dense material and the angle of incidence is greater than the critical angle
54
at the boundary between 2 materials where n1 > n2 (higher refractive index to lower refractive index)...
- intensity of partially reflected ray increases
- angle of refraction approaches 90 degrees faster than angle of incidence
- (white light) dispersion becomes more obvious in the refracted ray
55
(total internal reflection) what happens if a light ray is incident at the boundary at the critical angle?
the reflected ray is perpendicular to the normal
56
how do you calculate the critical angle where the angle of refraction is less than 90 degrees where total internal reflection happens?
sinθc = n2/n1
where:
n2 < n1
θc < 90 degrees
57
what is an optical fibre + what are the uses?
its a thin piece of flexible glass that uses total internal reflection to transmit information encoded in light over long distances + around corners
uses:
- communication (eg. phone/TV signals) - carries more info than electricity
- medical endoscopes - see down the patient + flexible so won't injure the patient
58
what is the structure of an optical fibre?
- core: inner layer where pulses (infrared) are transmitted - made of very pure glass
- cladding: glass layer with lower refractive index than core - protects core from damage + prevents light leaking out
- coating: layer that prevents damage to the fibre
59
how do you get the highest bandwidth on an optical fibre cable ?
light rays need t take the shortest path along the cable - the straighter the path the better, zigzag = :(
60
why is it important that the cladding has only a small difference smaller in refractive index as the core?
- the critical angle at the core-cladding boundary is large
- light incident here below the critical angle escapes from the core - causes a loss in signal strength but avoids signals taking long zigzag paths
61
what is attentuation?
the weakening of signals due to losses as it travels
62
what is modal dispersion? (signal degradation)
- if a core is wide enough, rays of light entering will take different paths through the fibre + take different amount of time to reach the destination which would ruin the message
- reduced by using monomodal fibres - the diameter of the core is so small waves can only take one path - straight through
63
what is material dispersion? (signal degradation)
- if white light is used for the signal, the different wavelengths in it travel at slightly different speeds - causes them to separate as they travel through the fibre = less bandwidth
- solved by using monochromatic light or a repeating station every km or so
64
what is pulse broadening? (signal degradation)
- when a signal pulse gets broader as it travels along the fibre
- reduces quality of the signal + leads to loss of info if the pulses overlap
65
what is scattering? (signal degradation)
- small variations in glass (eg. impurity) = light scatters from original path
- some light may hit the boundary below the critical angle = it leaves the fibre
- solution: use incredibly pure glass
66
what is absorption? (signal degradation)
- photons may interact with electrons in the glass - causes electron excitation
- electrons will de-excite - re-emitted photons may not be the same wavelength/travelling in the same direction
67
what are bending losses? (signal degradation)
- if an optical fibre is bent too sharply, light will be incident at the core-cladding boundary at less than the critical angle
- some of the light will be reflected while some will be refracted out of the core
