Cherenkov radiation
- nicolelyu812
- Feb 20, 2024
- 3 min read
Updated: Feb 25, 2024
This is what you might see around an extra-abstract nuclear reactor (sorry about the abstract-ism). A ghostly blue hue surrounds the plant, eerie looking but nevertheless intriguing. This is Cherenkov radiation.
There are plently of explanations online - caused by particles over the speed of light, light equivalent of the sonic boom, power puff of tropical coconutettes.... ok maybe not that one. What is Cherenkov radiation anyway?
What is Cherenkov radiation?
Cherekov radiation is a blue-looking electromagnetic radiation. In fact, it would be inaccurate to say it is blue, since it atually covers also violet and the ultraviolet part of the spectrum (we don't see violet because our cone cells are more sensitive to blue). Cherenkov radiation forms a cone of wavefronts in its medium behind the particle it's coming from, thus it is isotropic - different when viewed at different angles.
What causes Cherenkov radiation?
Simply put, Cherenkov radiation is emitted due to charged particles travelling faster than the speed of light. But wait... didn't some physics law prohibit speeds above the speed of light? Let's break it down. For this scenario to happen to ordinary matter, we can:
a) Speed stuff up till it exceeds c, or
b) Slow light down beneath c
"c" is the top speed of the universe (which just happened to be the speed of light in a vacuum), so it would indeed break physics laws to accerlate anything above it. Thus we go with b). In a medium, the phase velocity (the velocity of a wave crest/trough) can be slower than c, and travelling faster than the phase velocity of light is what is needed to produce Cherenkov radiation.
Ok, I got the conditions, but why is light involved? When the charged particle travel through the medium, the medium molecules get polarized and gets associated with an electric dipole. For instance, if an electron is travelling through water, the water would arrange themselves so the slightly electrically positive side of the molecule faces the electron.
As the electron travels, the electric field associated with the electron moves, thus the electric field that the water molecules experience change. The electric field exerts a force on the water molecule in a different direction to align them with the new electric field.
During this alignment, water molecules accelerate. The acceleration of electric dipole of the water molecules creates a changing electric field, which, in turn, creates a changing magnetic field (check Maxwell's equations). The changing magnetic field induces a changing electric field which... you get the point. Electro-magnetic-waves here they come.
Ok, now it seems like every charged particle travelling a medium should produce light, right? You are correct in that the light does exist. However, it won't be as noticeable as Cherenkov radiation. Here is where the "exceed the phase velocity of light" part comes in.
When the speed of the particle is relatively slow, the shell of wavefront would just dissipate its energy in the surrounding medium "individually". But when the speed of the particle is faster than the phase velocity of light, the wavefronts "bunch up" in a cone and accumulates in energy, so it's noticeable!
See how the wavefronts bunch up at the edges? That creates the light-equivalent of a sonic boom!!
We can also easily calculate the emission angle of the Cherenkov radiation θ (with respect to the direction of travel of the particle) like this:
AMAZING!!
References:
ChatGPT :)
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