What do you think when you hear the term radiation. Poisoned milk? Heavy metals that stick around for millions of years? Jumping under your desk and covering your head? Brain cancer causing cell phones?
When I think of radiation I think of light bulbs, old style radiators (notice any similarity between the words radiator and radiate?) or burning ants with a magnifying glass.
Radiation comes in many flavors, one broad category of which I would like to discuss today; electromagnetic radiation aka light.
Light is a wave. Some scientist will tell you that it is a particle. These scientist aren't wrong but thinking of light as a particle can get you in trouble if you don't have some grasp of the quantum mechanics. For the purposes of this discussion (and many to come) we will treat light as wave. This is at least as valid as treating it like a particle. The material below may help persuade you.
Light is called electromagnetic radiation or EM radiation for short. To understand light it helps to understand a little about electricity and magnetism.
Electricity and Magnetism
Two like charges repel one another and two opposite charges attract. Each charge is said to have its own electric field around it that affects the behavior of other charges in that field. This field drops off with an inverse square law. Inverse square laws are common in the field of radio transmission (incidently another type of radiation).
If you were standing next to fire and decided to move twice as far away from it as you have been standing how bright would the fire seem? If you answered half as bright, you are mistaken. If you said one quarter the initial brightness, then good for you, have a treat. The same goes with broadcast radio transmissions or star brightness or electric fields. Just about anything that disperses over three dimensional space will have to weaken as it moves away from the source. It will follow the inverse square law.
This electric field follows field lines that point radially toward or away from the charge (depending on convention and weather the charge is negative or positive). In physics convention field lines are continuous and magnitude is shown by the density of field lines. Calculus is somewhat different. The density, or strength, of field lines drops of with the square of the distance.
Magnetic fields are generally the result of current. Many models of magnetism are just hypothetical loops of current. To find the conventional magnetic field direction of current imagine wrapping you right hand around the current with your thumb pointed in the direction of conventional current. The Direction you fingers curls is the direction of the magnetic field. Magnetic fields lines are said to always be continuous. They are always perpendicular to electric field lines.
EM Waves
Imagine again our charged particle. So far I may have given the impression that field lines are infinite (if ever weakening) , but thats not entirely true. It takes time for a filed to "set up". Other charged particles wont "know" that another particle appeared (or disappeared for that matter) until the field lines reach them. This follows very well known speed limit of 300,000 km/s. For some reason this is called the speed of light but I think it might be more accurately named speed of field propagation. This same limit applies to magnetic fields, gravity fields and everything else in the universe.
Imagine for a moment that our particle is attached to some armature that can move it up and down very quickly. Imagine what is happening to field around the particle. The field cannot update instantaneously so the field near the particle moves in sync with the particle but further away it does not.
Now imagine the field as a kind of elastic fabric. Moving the particle generates circular ripples to propagate from it through the field "material".
Because it is a moving particle we can treat it as current and we can therefore predict magnetic fields as well as electric ones. The magnetic field is always perpendicular to the electric field. It will propagate in a ripple pattern at the exact same speed as the electric field.
Electro magnetic waves are nothing more than this, though they may vary in the way they are generated it generally boils down to oscillating electric charges.
Radio Waves
Radio waves are a type of EM radiation that is often the result man made devices. Current is a collection of moving charged particles. Alternating current is a type of current that switches direction many times a second. It is alternating current which transmits radio waves.
Because radio waves are ripples in electric and magnetic fields they can exert force on charged particles, even generate current. Wires that have their electrons moved by radio waves receive radio transmissions. This process is called electromagnetic induction.
The Electromagnetic Spectrum
EM waves - light can exist on virtually any frequency. At the low end in the bassists section we have radio waves which I already mentioned. But as we move up the orchestra of the universe there are -in order of increasing pitch- microwaves, infrared, visible light (red to blue), ultraviolet, x-rays and gamma rays.
Higher frequency light tends to be higher in energy than lower frequency light. Higher frequency also means a shorter wavelength.
Microwaves
Microwaves are just high frequency radio waves. The reason they are dangerous to humans and make a convenient way to heat your lunch is because of that peculiar Micky Mouse of molecules, the water molecule.
Water molecules are electric dipoles. I have not the foggiest notion why this is (actually its the Mickey Mouse shape that causes it but why that shape is a mystery) The hydrogen "ears" of the molecule are slightly positively charged and the oxygen "chin" is slightly negatively charged. This allows life to exist and snow flakes.
Because H2O is an electric dipole it is susceptible to forces being exerted on it through EM induction, particularly in the frequency of microwaves. When your microwave oven cooks a bowl of noodles it is using EM to shake just the water molecules. The moving water molecules then impart heat to everything else around them.
As an experiment try heating up water and oil simultaneously in separate glass dishes. Leave the water in for a few minutes and it will boil. The dish with the oil in it shouldn't be heated at all.
Infrared
There are three types of ways that warm objects transmit thermal energy to rest of the universe. They are called convection, conduction and radiation. For this blog in general we are mostly concerned with that last one.
Everything that has a temperature radiates thermal energy. For everyday things like people, animals and machines all of this happens in the range of infrared. Infrared goggles are what allow the special forces to spot tangos (bad guys) in the dead of night without the add of illumination. It is also what allows some predators to hunt at night. There is no trick that can hide this radiation except maybe an infinite heat sink or actually lowering your temperature to background levels.
Spaceships do not have the advantage of air circulation or underground water tables to keep cool. The only way a spaceship can release thermal energy is by radiation. If the ships radiators fail then the crew is in big trouble.
Visible light
Visible light is just a small sliver of the EM spectrum. It also contains the range that most of our suns energy is released at. Our eyes evolved to take advantage of this particular bath of radiation. I speculate that creatures that evolved around a red dwarf would see our star Sol as bluish and creatures that are born around large blue stars might see our sun as rather red.
Visible light is a higher frequency and therefore more energetic than all the types of radiation I have discussed so far. Less energetic forms of light have less ability to penetrate and damage tissue.
It makes less sense to talk about the cancer causing potential of cellphones (radio waves) than the cancer causing potential of light bulbs. I would say if you want to prevent cancer, live in a cave but there are reasons why that wouldn't even help anyways.
Blue light has a slightly higher frequency, and shorter wavelength, than red light.
Ultraviolet
Ultraviolet or UV light is what causes you to tan or it can cause skin cancer. It is what makes all your white threads light up when you visit the club or it allows the dentist to more clearly see how well you've been brushing your teeth.
UV rays cause certain substances to excite their outer most electrons. When the electrons fall to a lower energy state they release energy in the form of visible light. Its usually safe, but from here on out radiation becomes more dangerous.
X-rays
X-rays are usually created when high speed charged particles pass through a conductor. The charged particle slows down but all the while imparting unusually high amounts a energy to the surrounding metal. The metal immediately releases this energy in the form of x-rays.
When you go to the doctor to get an x-ray, the technician gives you a led bib and some reassuring words then steps behind a thick lead wall.
X-rays are a little dangerous. They are high in energy and short in wavelength. This allows them to penetrate solid objects. This in turn allows pictures to be taken of your beautiful insides. Other types of radiation bounce harmlessly off your skin but not x-rays.
For the most part you don't have to worry to much about x-rays. There is a difference between acute and chronic doses of radiation. The x-rays you get at the doctor are nowhere close to giving you an acute dose of radiation and because you only get x-rayed a few number of times in your whole life you are not likely to receive a chronic dose either.
For the technician it is a different story. If they stood next to every patient who got an x-ray while they got the x-ray they would receive a chronic dose of radiation. They would be at a high risk of cancer.
Gamma Rays
The deadliest of all, gamma rays are only produced in nuclear reactions or radioactive decay. Gamma rays are usually associated with alpha and beta radiation (alpha beta gamma get it?). Although alpha and beta radiation are produced in the same type of processes they are not EM radiation at all but particle radiation.
The atomic nucleus has different energy states just like the "orbital" electron shell. Gamma rays are the result of higher energy states collapsing inside the nucleus. Because electric forces are non negligible in their role inside the nucleus this collapse sends ripples into the EM fields around it.
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