Nature of Light Flashcards
What is an electromagnetic wave?
transverse electric and magnetic waves oscillating perpendicular to each other
acceleration of a charge
Why do electromagnetic waves form?
a changing in E field creates a changing M field which creates a changing E field until infinity
How can EM waves move through vacuums?
the wave self propagates
Who was James Clark Maxwell and what did he do?
physicist who unified electricity and magnetism into electromagnetism as he found similarities between these two fields and predicted a spectrum of light
He proved:
- light = EM wave with speed c (3x10^8 ms^-1)
- light can be polarised since they’re transverse waves
What do the 4 Maxwell Equations state?
- q/ε0 = total flux line in area (ε0 = vacuum permittivity)
- sum total of magnetic flux lines = 0
- voltage = EMF
- Ampere’s law w/ changing currents and areas considered
How did Olaf Romer find c?
comparing how long the closest possible Io eclipse took to how long the furtherest possible Io eclipse took, he deduced it to be 2.12 x 10ms^-1
It was off because of inaccurate measurements of the distance of the earth’s orbit at the time
How did James Bradley find c?
Bradley used stellar aberration (taking earth’s orbit into consideration on how to actually view a planet), comparing angles to find c to be 3.03 x 10ms^-1
How did Fizeau/Foucalt find c?
using a cog wheel machine, Fizeau found the time it took for a cog tooth to fully block light equal to the 16km/c (distance of 8km reflected back to 16km over speed c since t = d/v) and calculated it to be 3 x 10^8ms^-1
Foucault a collaborator, later used more refined methods to get a measurement 0.5% off modern c
Why are UV, X-rays and Gamma rays so dangerous?
These lights contain incredibly high amounts of energy condensed into small wavelengths which can easily damage organic material i.e. humans
What happens in an emission spectra?
a gas is excited, and whatever lights it emits is shone onto a spectroscope
It looks like a black background with thin strips of colour on it
What happens in an absorption spectra?
a gas is cooled, and a white light in shone in it. The gas will absorb some of this white light and the rest of it is shone onto a spectroscope
It looks like a complete ROYGBIV background with thin black strips on it
What relation does an emission spectra have to the absorption spectra?
The lines of light of an emission spectra fill in the blanks of the absorption spectra
This is because the thin lines of black represent the photons absorbed which later become emitted i.e. the thin lines of light of an emission spectra
What are the two types of spectroscopes?
Prism spectroscope: source goes through a prism and then goes into your eye!
Diffraction grating: a majority of modern ones are these. They are either reflected on a reflection grating or go through a transmission grating
What can scientists gauge on a star with a spectra?
structure chemical composition star velocity temperature density
What are the two models of light and who were they made by?
Christiaan Huygens - Wave model
Isaac Newton - Particle model
What did the wave model state?
- Light is propagated in ‘wavefronts’ from wavelets
- Reflected on plane surfaces
- Refracted on plane boundaries
What did the particle model state?
- Light is composed of ‘corpuscles’
- They travel in a straight line
- They speed up in denser mediums like glass because of attraction to the denser medium due to attractive forces (vector breakdown)
- Different corpuscle sizes mean different colours
- They are perfectly elastic
- Rigid
Why was the wave model disregarded in favour of the particle model in early optics?
Because Newton was a gee
What couldn’t the particle model explain?
- Denser medium speed thing (just wrong smh)
- Refraction
- Partial reflection
- Diffraction, interference, polarisation
When is diffraction most evident?
If the wavelength of the wave is same size as the slit it’s going through
What was Young’s double slit and what did his experiment prove?
Light source → single slit → double slit → wall
That light has wave properties because of how they construct and destruct at certain points when shone at a wall as they got diffracted. This interference pattern is a result of a path difference
What is the formula for Young’s double slit experiment?
y = mλL/d
y = length of centre of point on wall to m band m = no. bright/dark spots from centre λ = wavelength L = distance from slits to wall d = distance between slits
What is the path difference length (l)?
dsinθ = mλ = l
In a double slit, the angle from centre to point is equal to (mathematically)?
tanθ = y/L
sinθ = mλ/d
Draw a diagram of the double slit experiment
https://www.researchgate.net/profile/Michael-Mobley/publication/253374363/figure/fig1/AS:644285078728704@1530620941252/Diagram-of-Youngs-double-slit-experiment-indicating-the-intensity-of-light-striking-a.png
What is polarisation?
When light intensity reduces due to it going through a filter
What is unpolarised and polarised light?
Unpolarised: before it goes through a filter
Polarised: after it goes through a filter, has its additional waves removed
What are examples of polarising mediums?
Shades of sunglasses, Radio transmission
If light moves through a polarising filter, what will the intensity out be?
Iout = 0.5Iin
Formula for polarisation with two filters?
Iout = Iin (cosθ)^2
This is called Malus’ law
Types of polarisation?
Reflection and refraction:
reflection can remove a plane wave
refraction reduces intensity for perpendicular and parallel
e.g. light on a lake surface
Scattering:
light will be absorbed or re-radiated
reflection on surface of particle
e.g. light going through curtains going through dust
What is a blackbody?
A perfect emitter AND absorber of light that falls into it. Its colour indicating the wavelength it is absorbing/emitting
What is blackbody radiation?
The energy intensity of radiation that the blackbody emits
What are the order of colours of a blackbody?
None → Red → Orange → Yellow → White
rising intensity of energy, it becomes white because of the multiple light frequencies emitted all adding up
What was the classical model state would happen to blackbody radiation?
As the blackbody emitted radiation → temperature would increase into infinity because it is a continuous wave of energy
which is wrong experimentally
called the UV catastrophe
How did Plank solve the UV catastrophe?
He proposed light was a continuous stream of packets of energy called quanta, rather than a continuous stream of a wave
note quanta = specific amounts
What is the photoelectric effect?
The phenomena in which a light shone on something (usually a metal) causes electrons to be emitted
Why did the photoelectric effect cause trouble for the classical model of light?
- Intensity (yes but no)
- Emission time
- Frequency
- Energy
What did the classical model predict and what was the experimental data for emission time?
Classical prediction: Lower intensity meant significantly longer emission time
Experimental data: If emission occurred, it would always be essentially instant
What did the classical model predict and what was the experimental data for frequency?
Classical prediction: Emission of photoelectrons independent of frequency
Experimental data: Emission of photoelectrons only occurred if certain frequencies were reached or surpassed
What did the classical model predict and what was the experimental data for energy?
Classical prediction: Higher intensity = Higher energy in electrons (ALWAYS)
Experimental data: Energy of electron will increase UNTIL reaching a maximum energy point and followed the formula Emax = hf - φ
Where hf is the energy of the incident photon and φ is the least amount of energy it needs and Emax is the energy of the electron emitted
What was the one feature that was predicted correctly (to some extent) by the classical model of light?
Intensity: Both classical predictions and experimental results stated that higher intensity means higher photocurrent
BUT experimental data shows that a photocurrent can only occur under certain frequencies
What is the threshold frequency and what does it look like on a Energy-frequency graph?
The smallest frequency (fo) necessary for photoemission
It is the x-intercept on a Energy-Frequency graph
What is the work function and what does it look like on a Energy-frequency graph?
The smallest energy (φ) necessary for photoemission
It is the y-intercept on a Energy-Frequency graph AND it is ALWAYS under 0 (but the φ has a positive value)
Energy is dependent on what?
Frequency, because Ep = hf
Intensity is independent of what?
Energy (of single photon), because Ep = hf
Intensity means the no. photons in a given area, it doesn’t say anything about how much energy is in the photons
How can you calculate the total energy of something if you are given the number of photons and the energy of the individual photons?
Et = nEp
Et = total energy n = no. photons Ep = energy of individual photon = hf
What are Einstein’s two postulates?
- Inertial frameworks are the same for everyone
2. c is constant for everyone doesn’t matter how fast someone or something is moving
What is proper time (to)?
The time in which the near light speed event is occurring
What is dilated time (t) and how do you find it?
The time that is increased
t = to/√(1-(v^2/c^2))
When a stationary person A observes another person B going very fast, how will these people’s time appear to look like to each other?
Slower compared to themselves
What is proper length (lo)?
The length of un distorted thing
What is contracted length (l) and how do you find it?
The length that is shrunken down
L = Lo x √(1-(v^2/c^2))
When a stationary person A observes another person B going very fast, how will these people’s lengths appear to look like to each other?
Shortened
What is rest mass (mo)?
The mass of something stationary
What is relativistic mass (m) and how do you find it?
The mass of something moving at near light speeds
m = mo/√(1-(v^2/c^2))
When a stationary person A observes another person B going very fast, how will these people’s mass appear to look like to each other?
Heavier (more mass)
What does E = mc^2 state pretty much?
Mass and energy are the same thing shown differently
What is the more technically correct version of the energy-mass equivalence equation?
E = 1/2mv^2 +moc^2
What is an inertial frame of reference?
When you are stationary or moving at a constant linear velocity i.e. you are not accelerating/rotating
and where Newton’s 1st law is obeyed
you FEEL stationary
What was the Michaelson-Morley experiment?
An experiment to determine the existence of the aether
What was the Michaelson-Morley experiment’s method?
They spun a wheel that shone light on a half reflecting mirror at 90°. When the two light beams met, they should have an interference pattern different to if it weren’t spun
What did the Michaelson-Morley experiment end with?
They couldn’t conclude anything from the result i.e the null hypothesis
What was the problem with measuring the speed of light before 1950?
Machines were not precise enough to measure at such short times or lengths
How many equations did Maxwell have initially?
He had 20 but they were cut down to just 4
What is Hertz’s contribution to discovering the properties of light?
He found ‘invisible’ radio waves from spark gaps which behaved like visible light (same speed)
Gave credence to Maxwell’s theory
What spectra is typically used for stars and their temperature?
Absorption spectrum
What is a red shift and a blue shift?
The spectra measured will move to the red side if it’s travelling away from us and to the blue side if it’s moving towards us
All hot objects no matter composition do what?
Emit radiation
Where do ‘cooler things’ and ‘warmer things’ go on a blackbody graph respectively?
cool = blue side warm = red side
What are these blackbody curve graphs, graphs of?
https://preview.redd.it/699cgiciig371.jpg?width=640&crop=smart&auto=webp&s=4430689249a1b5d213eb6946b35c03941ef3b6c8
https://preview.redd.it/ezetwjwaig371.jpg?width=960&crop=smart&auto=webp&s=c3979cc0913b920733cf5a7bb6d5d72adafe30b2
Diffraction gratings… What are they and why?
They’re light shone through a number of really thin gaps
They create a pattern that’s white, then a number of inverse prisms the gaps that widen as they go
https://www.newport.com/mam/celum/celum_assets/Figure_79-Photonics_Handbook_800w.jpg
What is so cool about diffraction gratings?
They are much more precise compared to double slits
With two polarising filters, what are the first and second filters called?
Polariser
Analyser
Formula for Double Slit Experiment?
y = mLλ/d
[y = distance from centre (m)] [L = distance from screen (m)] [d = distance between slits (m)]
Formula for Wave Difference?
d sinθ = mλ
[d = distance between slits (m)]
Formula for Polarisation?
Io = Ii (cosθ)^2
note Ii is the light before the analyser and Io is the light after the analyser and Ii = I/2 where I is unpolarised light (before polariser)
Formula for Maximum Kinetic Energy of Photoelectron?
Kmax = qVstop = 1/2mv^2 Kmax = hf - Φ Kmax = h(f - fo)
[Φ = hfo (fo = minimum frequency)] [h = 6.626 x 10^-34]
Formula for Kinetic Energy of Photon?
E = nhf
[n = number of photons]
note n is usually just 1 unless stated otherwise i.e. total energy will require you to use E = nhf
Formula for Wien’s Law?
λmax = b/T
[λmax = greatest wavelength (m)] [G = wien's constant (2.898x10^-3)]
Formula for Time Dilation?
t = to/√(1 - (v^2/c^2))
[to = time in perspective of fast moving thing (secs)] [t = time observed by slow fellas (secs)] [c = speed of light (3x10^8)]
Formula for Length Contraction?
L = Lo√(1 - (v^2/c^2))
[Lo = length of fast moving thing (m)] [L = length observed by slow fellas (m)] [c = speed of light (3x10^8)]
Formula for Relativistic Mass?
m = mo/√(1 - (v^2/c^2))
[mo = rest mass (kg)] [m = mass observed by slow fellas (kg)] [c = speed of light (3x10^8)]
Formula for Relativistic Momentum?
p = mov/√(1 - (v^2/c^2)) p = mv
[mo = rest mass (kg)] [m = relativistic mass (kg)] [c = speed of light (3x10^8)]
Formula for Mass-Energy Equivalence?
E = mc^2
[c = speed of light (3x10^8)]
