2019-05-23
Light is made of photons.
Light has both color and brightness. Color is related to the energy of photons and brightness is related to the number of photons.
Color determines how light interacts with objects. Black surfaces absorb photons of all visible colors, while white surfaces reflect them. Glass is transparent to visible light; many materials are transparent to radio waves.
Colors bluer than ultraviolet are called "ionizing" because photons at those energies can break molecules apart. For example, they can turn water into hydrogen gas and oxygen gas. Non-ionizing colors (like in your microwave oven) can only heat things up.
A dim ionizing light of one photon could break one water molecule. A bright ionizing light could make a pot of water appear to boil with hydrogen and oxygen bubbles.
All light is radiation. But "radiation" often means ionizing radiation and that's how I'll use it here.
Radiation exposure can be measured in sieverts (Sv). It's the amount of energy absorbed per kilogram of material.
Radiation can kill you by burning you to death. But a vastly smaller amount than would burn can still kill by breaking your DNA molecules.
Breaking other molecules doesn't do much, because the body is constantly making and breaking them anyway. That's what cells do. But they need the instructions in DNA to do it the right way. DNA molecules are the only molecules in the body where small changes can cause big problems.
Exercise and normal metabolism also break DNA, so cells have ways of repairing it. But they can only repair it so fast. If damage happens faster than that, cells can die.
Cells die all the time for various reasons. When they do, healthy cells divide to replace them. But they can only divide so fast. If too many cells die at once, you can die.
It's thought that low levels of radiation may not be harmful, or may even be beneficial (like exercise). The truth about it is currently unknown. But it's clear that the dose rate, like in Sv per hour, is more important than the total dose you might get over a long period of time.
Here are some dose rate examples in microsieverts per hour:
| location | uSv/h |
|---|---|
| my office | 0.1 |
| Denver, CO | 1.4 |
| typical domestic flight | 2.5 |
| beach resorts in Guarapari, Brazil | 50 |
A dose of 4,000,000 uSv = 4 Sv in one day is lethal about half the time.
The ratio between that and a typical trip on Southwest Airlines is about 4,000,000/24 * 1/2.5 = 70,000.
The ratio between lethal and effective doses of alcohol is about 15.
If ionizing radiation were visible, 4 Sv/day would look like 3 lux, which is the illumination you get from two candles about a meter away.
The power output of a nuclear reactor is determined by its fission rate (the rate it breaks uranium atoms), just like the power of a gas engine is determined by the rate it burns gas (breaks gasoline molecules).
Each fission emits roughly the same amount of radiation, so a 6 megawatt reactor emits twice as much as a 3 megawatt reactor.
Radiation intensity decreases with the square of your distance to the source. Doubling the distance lowers the dose rate by a factor of 4. Tripling the distance lowers it by a factor of 9, and so on. If the level falls to 40% when you walk 5 ft away, it will fall to 10% when you walk another 5 ft away.
That's because rays spread out as they travel. You can imagine radiation spreading out like a sphere around the source. You only experience the part of the sphere that hits you. If you're farther away, the sphere is bigger and less of it hits you. The surface area of a sphere grows like the square of its radius.
Distant sounds are quieter and distant candles are dimmer in exactly the same way (though our senses have powerful compressors that try to compensate for it... the pupils in our eyes are one example). Even gravity obeys this "inverse square law".
The inverse square law applies in a vacuum like outer space. Radiation intensity can decrease even faster if there's air or another material around the source, because rays could be absorbed before they reach you. That's called "shielding".