by Ciro Santilli (@cirosantilli, 32)
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.
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.
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.
Video 1.
The Story of Light by Bell Labs (2015)
. Source. Gives some ideas of the history of fiber optics. Features: Herwig Kogelnik.
Video 2.
Fiber optic cables by EngineerGuy
. Source.
These are closely related to lasers, as they do a similar basic job: take a DC source as input and amplify light. Lasers just happen to use the input voltage to also generate the incoming light.
These are pretty cool, they are basically a laser
This one was a huge advance it seems.
Video 1.
Erbium-doped fiber amplifier by Millennium Technology Prize
. Source.
It's the thing that allows you to connect fiber optics into a compter, or the corresponding port for the thing.
Many of them can take two fibers as input/output because fiber optics cables often come in pairs because it is needed for duplex.
Video 1.
How to choose SFP transceiver for fiber optical cable by FASTCABLING
. Source.
From a practical point of view single-mode:
  • upside: can go further without a repeater. In multi-mode optical fiber, different modes travel at different speeds, and start interfering with each other at some point
  • downside: lower bandwitdh, because we can fit less modes into it
As such, typical applications are:
From a mathematical point of view:
Figure 2.
2009 Nobel Prize lecture
. Poor Charles was too debilitated by Alzheimer's disease to give the talk himself! But if you've got a pulse, you can get the prize, so all good.

City of Light: The Story of Fiber Optics (1999)

The book is a bit slow until Charles K. Kao comes along, then it gets exciting.
Video 1.
Donated Eskalab Spectrophotometer by CuriousMarc
. Source.
Bibliography:
Video 1.
Replicating the Fizeau Apparatus by AlphaPhoenix (2018)
. Source. Modern reconstruction with a laser and digital camera.
Video 2.
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...
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.
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 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 efficiently of this conversion and output spectrum of this conversion on some cheap device we can buy today.
Video 1.
Magnetron, How does it work? by Lesics (2020)
. Source.
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 1.
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.
Video 1.
How Microwaves Work by National MagLab (2017)
. Source. A bit meh. Does not mention the word cavity magnetron!
Video 2.
How a Microwave Oven Works by EngineerGuy
. Source. Cool demonstration of the standing waves in the cavity with cheese!
420 to 680 nm for sure, but larger ranges are observable in laboratory conditions.
Figure 1.
Toshiba D-088 dental X-ray tube
. Source.
Video 1.
William Coolidge explains medical imaging and X-rays (1940).
. Source. Video sponsored by General Electric. A cool insight of this video is that a hot cathode is a more reliable electron source. Previous systems, and presumably including the discovery of X-rays, leftover gas in the tube was used. But this makes things more difficult to control, as we also want to remove as much gas as possible from the vacuum, otherwise electrons collide with the gas and lose energy before hitting the anode.
Video 2.
How Does X ray Tube Works by BiomedEngg
. Source. Describes in particular the rotating cathode method. Interesting observation that this is especially important since the cathode cannot cool quickly due to the vacuum.
Original 1931 experiment by Raman and Bhagavantam: dspace.rri.res.in/bitstream/2289/2123/1/1931%20IJP%20V6%20p353.pdf
Experimental setup to observe radiation pressure in the laboratory.
Application of radiation pressure.
First live example: en.wikipedia.org/wiki/IKAROS
Figure 1.
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.
Bibliography:
Video 1.
How to use an SiPM - Experiment Video by SensLTech (2018)
. Source.
Video 2.
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 1.
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 2.
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 1.
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 2.
Quantum Optics - Beam splitter in quantum optics by Alain Aspect (2017)
. Source. More theoretical approach.
Video 3.
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.
Can be used to detect single photons.
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:
The science and engineering of light!
When dealing more specifically with individual photons, we usually call it photonics.
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.
The most important type of lens is the biconvex spherical lens.
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:
    (1)
    where and are the two distances.
If you pass parallel light.
For a biconvex spherical lens, it is given by:
(1)
where:
  • n: f nidnex
Video 1.
Carl Zeiss, Explained by Asianometry (2021)
. Source.
Video 2.
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.
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.
The science and engineering of photons!
A bit more photon-specific than optics.
Video 1.
Silicon Photonics: The Next Silicon Revolution? by Asianometry (2022)
. Source.
Video 2.
Running Neural Networks on Meshes of Light by Asianometry (2022)
. Source.
Video 3.
Silicon Photonics for Extreme Computing by Keren Bergman (2017)
. Source.

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  1. Elementary particle
  2. Subatomic particle
  3. Standard Model
  4. Particle physics
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