# What do we know about photon? (light)

I should at some time try to be a little more comprehensive than what will follow, I have elsewhere said how photon is very special: zero mass, vector spin state, fastest particle, force carrier of electromagnetic force — actually Maxwell waves and so on.

We can not talk about the position of the photon in relativistic quantum mechanics — we do not have a complete consistency but still. Also photon is it’s own anti-particle …

I was just reviewing a little about the photon: in 1600 Galileo was trying to measure the speed of light, at that time light speed was thought to be infinite, that is; instantaneous. The light is there in alpha-centauri, and it is there in here, at the same instant.

Galileo took his postdoc (pun) to a distant place and took a light source and uncovered it so his postdoc can see it remotely, then he would uncover his, upon seeing this and Galileo would see that and note his time.

ha ha ha that’s silly Galileo, Galileo’s ghost: yes Mohan, easy for you to say that, how about detecting dark matter? Xenon, ha ha ha ha ha ha … Xenon will detect the dark matter, ha ha hahhh haah a

Needless to say Galileo failed or rather he measured light speed to be infinite in line with the PDG (pun) publication of 1600.

A few years later (1849) Fizeu (France) performed his experiment. This was the first experiment that provided the correct result. Compare to the most precise $2.99 \times 10^8 \frac{m}{s}$ of today to what Fizeu obtained: $3.12 \times 10^8 \frac{m}{s}$.

Amazing considering he set it up like this: mirrors and reflectors about 9 km apart. The light near the source would be either stopped or allowed by a motor, that rotates fast and has 720 teeth. The teeth blocks light and the next kink allows it, so from a properly calibrated motor speed, a resulting maximum intensity and the distance, light has to travel, he obtained a value which is so close to the actual value.

Foucault was in collaboration with Fizeu but they fought over what method to use to chop light into pulses. Foucault and Fizeu separated and Foucault came up with his measurement of light speed in water, where he used a tube of water in one of the path in duplication.

~75 years later Michelson — the first American Physicist to win a Nobel prize of the Michelson-Morley fame, obtained the speed of light in various media — air, water etc, and was very precise.

I need to review more on that if you are interested.

It turns out that light has a smaller speed if the matter is denser than vacuum where it has the highest speed. In water the speed of light is about 3/4th of that in vacuum. In glass 2/3rd of that in vacuum — which is less than in water, and in air only 87 km/s less — out of 300,000 km/s.

In mediums therefore a refractive index is defined, which is how much faster the light travel in vacuum compared to any media.

When I review more about light, I promise to tell you more.

Categories: light, optics, photon, Physics