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Initially light was though of as a wave because it experienced interference as shown by experiments such as:
But then, some key experiments also start suggesting that light is made up of discrete packets:
and in the understanding of the 2020 Standard Model the photon is one of the elementary particles.
This duality is fully described mathematically by quantum electrodynamics, where the photon is modelled as a quantized excitation of the photon field.

Light

words: 821 articles: 49

Wave-particle duality

words: 110 articles: 9
The history of light if funny.
First people thought it was a particle, as per corpuscular theory of light, notably Newton supported the corpuscular theory of light.
But then evidence of the diffraction of light start to become unbearably strong, culminating in the Arago spot.
And finally it was undertood from Maxwell's equations that light is a form of electromagnetic radiation, as its speed was perfectly predicted by the theory.
But then evidence of particle nature started to surface once again with the photoelectric effect. Physicists must have been driven mad by all these changes.

Corpuscular theory of light

words: 30 articles: 1
The Quantum Story by Jim Baggott (2011) page 2 mentions how newton's support for the corpuscular theory of light led it to be held for a very long time, even when evidence of the wave theory of light was becoming overwhelming.

Light source

articles: 5

Lamp

articles: 4
Gas-discharge lamp
articles: 2

Optical fiber

words: 17 articles: 3
Video 1. The Story of Light by Bell Labs (2015) Source. Gives some ideas of the history of fiber optics. Features: Herwig Kogelnik.

Speed of light

words: 203 articles: 7
Experiments: speed of light experiments.

Speed of light experiment

words: 52 articles: 1
Bibliography:
Video 2. Replicating the Fizeau Apparatus by AlphaPhoenix (2018) Source. Modern reconstruction with a laser and digital camera.
Video 3. Visualizing video at the speed of light - one trillion frames per second by MIT (2011) Source. Fast cameras. OK, this takes it to the next level.
It is so mind blowing that people believed in this theory. How can you think that, when you turn on a lamp and then you see? Obviously, the lamp must be emitting something!!!
Then comes along this epic 2002 paper: pubmed.ncbi.nlm.nih.gov/12094435/ "Fundamentally misunderstanding visual perception. Adults' belief in visual emissions". TODO review methods...

Faster-than-light (FTL)

words: 97 articles: 3
In special relativity, it is impossible to travel faster than light.
One argument of why, is that if you could travel faster than light, then you could send a message to a point in Spacetime that is spacelike-separated from the present. But then since the target is spacelike separated, there exists a inertial frame of reference in which that event happens before the present, which would be hard to make sense of.
Even worse, it would be possible to travel back in time:
Figure 1. Spacetime diagram illustrating how faster-than-light travel implies time travel. Legend an explanation are shown in this answer.
Bibliography:

Electromagnetic spectrum

words: 491 articles: 18
Notably used for communication with submarines, so in particular crucial as part of sending an attack signal to that branch of the nuclear triad.
This is likely the easiest one to produce as the frequencies are lower, which is why it was discovered first. TODO original setup.
Also because it is transparent to brick and glass, (though not metal) it becomes good for telecommunication.
Some notable subranges:
Micro means "small wavelength compared to radio waves", not micron-sized.
Microwave production and detection is incredibly important in many modern applications:
Microwave source
words: 75 articles: 2
Microwave only found applications into the 1940s and 1950s, much later than radio, because good enough sources were harder to develop.
One notable development was the cavity magnetron in 1940, which was the basis for the original radar systems of World War II.
Apparently, DC current comes in, and microwaves come out.
TODO: sample power efficienty of this conversion and output spectrum of this conversion on some cheap device we can buy today.
Video 4. Magnetron, How does it work? by Lesics (2020) Source.
Microwave transmission
words: 59 articles: 2
Finance is a cancer of society. But I have to admit it, it's kind of cool.
arstechnica.com/information-technology/2016/11/private-microwave-networks-financial-hft/ The secret world of microwave networks (2016) Fantastic article.
Video 5. Lasers Transmit Market Data and Trade Execution by Anova Technologies (2014) Source. Their system is insane. It compensates in real time for wind movements of towers. They also have advanced building tracking for things that might cover line of sight.
www.quora.com/Why-are-microwaves-used-by-satellites-rather-than-radio-waves
Microwave oven
words: 16
Video 6. How Microwaves Work by National MagLab (2017) Source. A bit meh. Does not mention the word cavity magnetron!
420 to 680 nm for sure, but larger ranges are observable in laboratory conditions.

Photon spin

words: 7
physics.stackexchange.com/questions/73942/how-do-we-know-photons-have-spin-1
Original 1931 experiment by Raman and Bhagavantam: dspace.rri.res.in/bitstream/2289/2123/1/1931%20IJP%20V6%20p353.pdf

Radiation pressure

words: 23 articles: 2
Experimental setup to observe radiation pressure in the laboratory.

Solar sail

words: 14
Application of radiation pressure.
First live example: en.wikipedia.org/wiki/IKAROS
Figure 2. A 1:64 scale model of the IKAROS spacecraft. Source.
You can't get more direct than this in terms of proving that photons exist!
The particular case of the double-slit experiment will be discussed at: single particle double slit experiment.
Production:
Detectors are generally called photomultipliers:
Bibliography:
Video 7. How to use an SiPM - Experiment Video by SensLTech (2018) Source.
Video 8. Single-photon detectors - Krister Shalm by Institute for Quantum Computing (2013) Source.
Phenomena that produces photons in pairs as it passes through a certain type of crystal.
You can then detect one of the photons, and when you do you know that the other one is there as well and ready to be used. two photon interference experiment comes to mind, which is the basis of photonic quantum computer, where you need two photons to be produced at the exact same time to produce quantum entanglement.
Video 9. One Photon In, TWO Photons Out by JQInews (2010) Source.
Mentions that this phenomena is useful to determine the efficiency of a single photon detector, as you have the second photon of the pair as a control.
Also briefly describes how the input energy and momentum must balance out the output energy and momentum of the two photons coming out (determined by the output frequency and angle).
Shows the crystal close up of the crystal branded "Cleveland Crystals Inc.". Mentions that only one in a billion photon gets scattered.
Also shows a photomultiplier tube.
Then shows their actual optical table setup, with two tunnels of adjustable angle to get photons with different properties.
Video 10. How do you produce a single photon? by Physics World (2015) Source.
Very short whiteboard video by Peter Mosley from the University of Bath, but it's worth it for newbs. Basically describes spontaneous parametric down-conversion.
One interesting thing he mentions is that you could get single photons by making your sunglasses thicker and thicker to reduce how many photons pass, but one big downside problem is that then you don't know when the photon is going to come through, that becomes essentially random, and then you can't use this technique if you need two photons at the same time, which is often the case, see also: two photon interference experiment.
The basic experiment for a photonic quantum computer.
Can be achieved in two ways it seems:
Animation of Hong-Ou-Mandel Effect on a silicon like structure by Quantum Light University of Sheffield (2014): www.youtube.com/watch?v=ld2r2IMt4vg No maths, but gives the result clear: the photons are always on the same side.
Video 11. Quantum Computing with Light by Quantum Light University of Sheffield (2015) Source. Animation of in-silicon single photon device with brief description of emitting and receiving elements. Mentions:
Video 12. Quantum Optics - Beam splitter in quantum optics by Alain Aspect (2017) Source. More theoretical approach.
Video 13. Building a Quantum Computer Out of Light by whentheappledrops (2014) Source. Yada yada yada, then at youtu.be/ofg335d3BJ8?t=341 shows optical table and it starts being worth it. Jacques Carolan from the University of Bristol goes through their setup which injects 5 photons into a 21-way experiment.

Photomultiplier

words: 53 articles: 2
Can be used to detect single photons.
Richard Feynman likes them, he describes the tube at Richard Feynman Quantum Electrodynamics Lecture at University of Auckland (1979) at one point.
It uses the photoelectric effect multiple times to produce a chain reaction.
Here is a vendor showcasing their device. They claim in that video that a single photon is produced and detected:
Concrete device described at: Video 7. "How to use an SiPM - Experiment Video by SensLTech (2018)".
A squeezed coherent state of light.

Optics

words: 1k articles: 42
The science and engineering of light!
When dealing more specifically with individual photons, we usually call it photonics.

Optical component

words: 258 articles: 10

Collimator

words: 34 articles: 2
Collimated beam
words: 34 articles: 1
Parallel light
words: 34
Often just called collimated light due to the collimator being the main procedure to obtain it.
However, you move very far away from the source, e.g. the Sun, you also get essentially parallel light.
Bibliography:

Lens

words: 224 articles: 2
The most important type of lens is the biconvex spherical lens.
Biconvex spherical lens
words: 215 articles: 1
Focal length
Each side is a sphere section. They don't have to have the same radius, they are still simple to understand with different radiuses.
The two things you have to have in mind that this does are:
  • converges parallel light to a point at center at distance known as the focal length.
    This is for example why you can use lenses to burn things with Sun rays, which are basically parallel.
    Conversely, if the input is a point light source at the focal length, it gets converted into parallel light.
  • image formation: it converges all rays coming from a given source point to a single point image. This amplifies the signal, and forms an image at a plane.
    The source image can be far away, and the virtual image can be close to the lens. This is exactly what we need for a camera.
    For each distance on one side, it only works for another distance on the other side. So when we set the distance between the lens and the detector, this sets the distance of the source object, i.e. the focus. The equation is:
    where and are the two distances.
If you pass parallel light.
For a biconvex spherical lens, it is given by:
where:
  • n: f nidnex

Optics vendor

words: 46 articles: 2

Carl Zeiss AG (ZEISS)

words: 46 articles: 1
Video 14. Carl Zeiss, Explained by Asianometry (2021) Source.
Video 15. How Carl Zeiss Crafts Optics for a $150 Million EUV Machine. Source. Difficulty: light at those frequencies get absorbed by lenses. So you have to use mirrors instead.
Carl Zeiss SMT
words: 15
Subsidiary of Carl Zeiss AG and also part owned by ASML, sole optics vendor of ASML as of 2020.
Can be approximated with a diaphragm.

Photonics

words: 755 articles: 23
The science and engineering of photons!
A bit more photon-specific than optics.

Silicon photonics

words: 83 articles: 4
Video 16. Silicon Photonics: The Next Silicon Revolution? by Asianometry (2022) Source.
Video 17. Running Neural Networks on Meshes of Light by Asianometry (2022) Source.
Video 18. Silicon Photonics for Extreme Computing by Keren Bergman (2017) Source.
Optical computer
words: 12 articles: 3
www.crunchbase.com/organization/inorganic-intelligence
Funding:
www.crunchbase.com/organization/lightmatter-inc
Presented e.g. at youtu.be/t0yj4hBDUsc?t=456 from Video 16. "Silicon Photonics: The Next Silicon Revolution? by Asianometry (2022)".
Lumai
words: 4
Funding:

Photon polarization

words: 400 articles: 8
The knowledge that light is polarized precedes the knowledge of the existence of the photon, see polarization of light for the classical point of view.
The polarization state and how it can be decomposed into different modes can be well visualized with the Poincaré sphere.
One key idea about photon polarization is that it carries angular momentum. Therefore, when an electron changes orbitals in the Schrödinger equation solution for the hydrogen atom, the angular momentum (as well as energy) change is carried out by the polarization of the photon!
Video 19. Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013) Source.
  • clear animations showing how two circular polarizations can make a vertical polarization
  • a polarizer can be modelled bra operator.
  • light polarization experiments are extremely direct evidence of quantum superposition. Individual photons must be on both L and R states at the same time because a V filter passes half of either L or R single photons, but it passes all L + R photons
This section discusses the pre-photon understanding of the polarization of light. For the photon one see: photon polarization.
polarization.com/history/history.html is a good page.
People were a bit confused when experiments started to show that light might be polarized. How could a wave that propages through a 3D homgenous material like luminiferous aether have polarization?? Light would presumably be understood to be analogous to a sound wave in 3D medium, which cannot have polarization. This was before Maxwell's equations, in the early 19th century, so there was no way to know.
Polarizer
words: 166 articles: 6
A device that modifies photon polarization.
As mentioned at Video 19. "Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013)", it can be modelled as a bra.
Fresnel equations
articles: 1
Good overgrown section in the middle of Fresnel's biography: en.wikipedia.org/w/index.php?title=Augustin-Jean_Fresnel&oldid=1064236740#Historical_context:_From_Newton_to_Biot.
Particularly cool is to see how Fresnel fully understood that light is somehow polarized, even though he did not know that it was made out of electromagnetism, clear indication of which only came with the Faraday effect in 1845.
spie.org/publications/fg05_p03_maluss_law:
At the beginning of the nineteenth century the only known way to generate polarized light was with a calcite crystal. In 1808, using a calcite crystal, Malus discovered that natural incident light became polarized when it was reflected by a glass surface, and that the light reflected close to an angle of incidence of 57° could be extinguished when viewed through the crystal. He then proposed that natural light consisted of the s- and p-polarizations, which were perpendicular to each other.
Malus' Law
words: 20 articles: 1
Matches the quantum superposition probability proportional to the square law. Poor Étienne-Louis Malus, who died so much before this was found.
physics.stackexchange.com/questions/61918/three-polarizers-45-apart/261188#261188
A more photon-specific version of the Bloch sphere.
In it, each of the six sides has a clear and simple to understand photon polarization state, either of:
  • left/right
  • diagonal up/diagonal down
  • rotation clockwise/counterclockwise
The sphere clearly suggests for example that a rotational or diagonal polarizations are the combination of left/right with the correct phase. This is clearly explained at: Video 19. "Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013)".
Figure 3. Poincare sphere. Source.

Photonics equipment

words: 202 articles: 7
An optical multiplexer!
Video 20. Control Light with Sound! by Les' Lab (2021) Source.
Interferometer
words: 107 articles: 2
Video 21. Fabry Perot Interferometer by JFC UCL (2016) Source. Description only, reasonable animations. Considers the case of two nearby beam splitters.
Video 22. Fabry-Perot Introduction by Williams College Physics (2020) Source.
Shows a working device. Confocal optical cavity, one of the mirrors scans back and forward moved by a piezoelectric motor, this is called a "scanning Fabry-Perot interferometer".
Does not produce an interference pattern, only an on/off blob, which is then fed into an oscilloscope for analysis. The oscilloscope shows both the mirror displacement (which is given by a voltage) and the light detector output.
Video 23. Aligning a Scanning Fabry-Perot Interferometer by University of South Florida (2016) Source. Very detailed, and shows of some nice equipment and techniques.
Figure 4. Source.
Optical fibre
words: 17
Video 24. The Story of Light by Bell Labs (2015) Source. A ultra quick and Bell Labs focused overview of the development of optical fibre.
Optical table
words: 67
The breadboard of photonics!
For example, that is how most modern microscopes are prototyped, see for example Video "Two Photon Microscopy by Nemonic NeuroNex (2019)".
This is kind of why they are also sometimes called "optical breadboarbds", since breadboards are what we use for early prototyping in electronics. Wikipedia however says "optical breadboard" is a simpler and cheaper type of optical table with less/no stabilization.
Video 25. A simple refracting telescope built on an optical bench by plenum88 (2013) Source.

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