the Cherenkov Effect

The effect discovered by 1958 Physics Nobel Prize laureate Pavel Cherenkov, caused by electrically charged particles in atoms moving faster than the speed of light in certain mediums. The effect is visible to humans as a blue glow.

Cause and Symptoms

In a vacuum, nothing moves faster than the speed of light. However, in some mediums, the speed of light slows down. In water, for example, light moves at 75% of its normal speed. If other particles do not slow down this much, they will continue to move faster than the speed of light, and will emit a blue glow.
A magician or volshebnik is able to control their alpha particles and fire them at other particles in order to generate radiation. The effort of fission and fusion causes the magic user to heat up and sweat, and this sweat evaporates rapidly due to the magician/volshebnik's high body temperature, creating a highly humid layer just above the magic user's skin. This serves as a medium for a light blue color when these magic users are generating radiation.
This blue color is caused by photons traveling at high frequencies and short wavelengths, which the human eye perceives as blue or sometimes violet. A magician or volshebnik using extremely powerful magic will even stop glowing as their radiation moves into ultraviolet spectrum, which is invisible to the human eye.

Silence. Then more shifting. Her feet touch down onto the floor and the stall door opens. When she shuffles out of the stall, she does so with her arms in front of her like she’s carrying a tray, but her hands are in fists. I barely recognize her. She’s a new freshman, Becky something, and her hands are shaking, and—

A stream of blood runs out of one nostril, leaving streaks on her chin and shirt before spattering onto the floor.

My rad meter says 35.9.

Her eyes are bloodshot, blood-streaked, her face red like she’s sunburned, her nose running blood like a faucet, her hands faded blue in the fluorescent shine of the bathroom lights.

Treatment and Prognosis

He's a bigger man, head bald and skin peeling away in oozing flaps. His eyes are bright and wild, and he’s glowing gas-flame blue. There’s a pull in my bones, and everyone else’s too, like someone’s got their fingers in our bone marrow. My teeth feel loose in their sockets. There’s only one thing that causes a sucking feeling like that: acute magic activation.

The Cherenkov Effect is not itself an illness or affliction, but is instead a visible indication that someone is using magic above a certain level. The levels of radiation required to generate the effect are above what's considered the safe use zone, so when Cherenkov radiation is seen, it's a sign that this magician or volshebnik will become ill or may even die.
The severity of the radiation sickness following Cherenkov radiation can sometimes be determined by the color of the radiation. A lighter blue is indicative of lower levels of radiation, whereas more purple or violet colors indicate higher radiation levels. A magic user who stops emitting a color but who is still generating radiation is most likely undergoing detonation, and all living creatures in the vicinity should evacuate as far as possible, as fast as possible.

Cherenkov Effect by Midjourney

Cause
Medium-to-high levels of magic use performed in certain mediums.

Symptoms
Visibly glowing blue, violet, or purple.

Treatment
Cherenkov radiation will dissipate as radioactivity decreases.

Prognosis
Acute radiation sickness is likely, and Cherenkov radiation on the violet or ultraviolet spectrum may result in detonation.

Sources

1. IAEA.org, "What is Cherenkov Radiation?"
2. Stanford.edu, "An Introduction to Cherenkov Radiation"