What is the first thing you think of when you hear the word “radioactive”?
For many people, this word conjures up images of ominously glowing material. In the opening credits to The Simpsons, a running gag is Homer's mishandling of a glowing green bar of radioactive material.
As someone who works with a fair bit of radiation, I can tell you that most radioactive substances used in the lab don't glow. This is too bad. In addition to making it easier to identify radioactive substances, working with glowing stuff all day would be totally awesome!
Unfortunately, the radioactive material we deal with looks just like any other normal material. The only way to observe the radiation is by using a specialized detector, such as Geiger-Muller tube.
So where did the idea of glowing radioactive things come from? It turns out that that some radioactive substances do glow, with the results ranging from awesome to terrifying. The most ubiquitous radioactive “glow” that most people have seen in real life is in glow-in-the-dark watch hands.
The glow in this case comes from a phosphorescent material which is activated by the particles emitted from trace amounts of radioactive material, most likely gaseous tritium. While tritium is radioactive, the particles emitted are not strong enough to penetrate the tube containing the gas and phosphor, and the concentration is so low that even if the tube was broken, the gas would not pose a health risk.
A more sinister radioactive element previously used for lighting was radium. Discovered by Marie Curie in the late 1800's, radium was used by the US Radium Corporation in the early 1900's to produce luminous paint by mixing it with zinc sulfide. The resultant mixture gave off a faint green glow, and was used to paint everything from watch faces to gun sights.
Yet, it turns out the radiation from radium is far more dangerous to people. A sad and dramatic example of radium poisoning can be seen in the case of the “radium girls,” a group of factory workers at US Radium who painted watches and dials with this radioactive paint and, unaware of the extreme danger, developed radiation sickness from their exposure .
If a sample of material is extremely radioactive, the emitted radiation can ionize the air around the material, producing a blue-purple glow. It takes a lot of radiation for this to occur, however, and the samples that I work with in my lab are far below this threshold. A terrifying story of this occurrence comes from Los Alamos National lab shortly after World War II, during an experiment with the appropriately named “demon core.”  During this experiment, a scientist named Louis Slotin was performing an experiment with a plutonium core and a set of beryllium radiation reflectors.
As Slotin moved the top reflector reflector up and down using the tip of a screwdriver (!!), the amount of neutron radiation reflected back into the plutonium core caused it to approach a “critical” state where a chain reaction would occur. During one of these experiments, the screwdriver slipped and the top reflector slammed shut on the core, causing it to momentarily go critical. Observers saw a flash  as the intense burst of radiation ionized the nearby air. While Slotin thought quickly and was able to bat the reflector off the core with his hand, he had already received a lethal dose of radiation and died within a few weeks.
A more benign, and in my opinion the coolest, form of radiative glow comes from a phenomenon called Cerenkov radiation. The mathematical formalism behind this phenomenon is complicated, but the basic idea is simple. Although the speed of light in a vacuum is fixed (and cannot be exceeded), light in a medium other than vacuum moves slightly slower.
For example, in water (with an index of refraction of ~1.33) the speed of light is reduced to 3/4 the speed of light in a vacuum. This means that if you got something moving through water faster than 3/4 the speed of light in a vacuum (say, an energetic decay particle from a radioactive element), that particle would be moving faster than light through the water! Similarly to how a fighter jet creates a sonic boom when it moves faster than the speed of sound, an energetic particle moving faster than the local speed of light in water will create its own shock wave, but with light instead of sound.
This shock wave manifests itself as bluish light. Since fission reactors use water to cool and moderate the nuclear fuel, this phenomenon can be readily observed in reactors as radiation from the core travels through the water around it (there are a bunch of awesome pictures of this online if you image search “Cerenkov radiation”).
By far though, the craziest form of Cerenkov radiation is observed in space. Astronauts from all the way back to the Apollo program reported seeing blue flashes when their eyes were closed. Since there is much more energetic background radiation in space than on Earth, Cerenkov radiation was proposed as the explanation, and experiments back on Earth confirmed that this was probably the case . So, essentially, energetic radiation from space was causing astronauts' eyes to glow.
Pretty far out!
 The whole situation was really messed up. Totally uninformed of the danger (which the company owners knew about), factory workers reportedly painted their nails with the radioactive paint, and were encouraged to straighten their paintbrushes with their lips. https://www.damninteresting.com/undark-and-the-radium-girls/
 This was dramatized in the movie “Fat Man and Little Boy”. If you want to see a spine-tingling re-enactment of the event, watch this clip from the movie -- https://www.youtube.com/watch?v=AQ0P7R9CfCY
 McNulty, P. J., V. P. Pease, and V. P. Bond. "Visual phenomena induced by relativistic carbon ions with and without Cerenkov radiation." Science 201.4353 (1978): 341-343. If you have journal access, you should totally read this study--it's nuts. In order to determine whether or not Cerenkov was responsible for the glow observed in astronauts' eyes, scientists did experiments on human subjects where the subjects looked into the beam of a particle accelerator. The beam energy was adjusted to be above and below the speed of light in water and the subjects mostly only reported seeing streaks of light when the beam energy was above the Cerenkov threshold, thus verifying that Cerenkov radiation was the probable cause of the phenomenon observed by astronauts.