So far the things I’ve written about in this blog have been about light behaving as “rays” — straight lines that bounce off of things or change direction when they pass through them. There is a bunch of other things that light does which has to do with the fact that light is really a wave. In this post I’m going to describe what kind of wave light is and what is “waving”. I’ll need to start with some perhaps unfamiliar and seemingly unconnected physics. So get ready for a bit of a longish detour!
Light is an electromagnetic wave — but what does electromagnetic mean? The word encompasses two things everybody has experienced. The first is electricity, the second magnetism.
Electricity is familiar — we are all know about plugging things into the wall socket. Lightning is another example (here’s a fantastic image from lightning’s Wikipedia article)
What causes the lightning bolt? Have you ever pulled off a sweater and had your hair go all frizzy and stick to it? Or touched something and got a little shock or spark? What about rubbing a balloon on your head and then sticking it to a wall? Or seen a little spark when plugging something in?
Even though most of the time you never notice it, all stuff is “charged” in one of two ways: either positively (+) or negatively (-). Positively charged things feel a repelling force between them and so do negatively charged things. Oppositely charged things feel an attractive force between them. This force is called the “electric” force. Most stuff has as much positive as negative charge in it — the overall charge is zero, i.e. the positive and negative charges cancel each other out. So a bowl and spoon don’t attract or repel each other because they each have as much + as – charge and so are effectively uncharged or “neutral”.
Any and every material — air, the earth, the keyboard I’m typing on, food, really just about anything in your day to day experience is made of atoms, and atoms have a positively charged core called a nucleus and a region of negatively charged things called electrons which are attracted to the nucleus and sort of stick to it, or around it.
Lightning happens when something causes the electrons on the surface of the earth to be stripped-off by the clouds — leaving the ground positively charged and the clouds negatively charged. The attractive force between the separated charges gets so strong that eventually something dramatic happens: a conduit is forced open where the charges can return to their proper locations. This conduit is the lightning bolt. The charges wreak havoc on the atoms in the air as they travel, causing them to get hot and glow.
OK — so we have learned that an electric force acts on charges. That’s one half of the physics of an electromagnetic wave. Now for the other half.
What’s magnetism all about? Probably the most familiar magnetic things are the fridge magnet and perhaps the compass needle. What was important for the electric force was charges, positive and negative ones. The magnetic force acts instead between the poles of other magnets (in fact it also acts on moving charges, but let me leave that aside for now as I think it will confuse things). The poles of magnets also come in two varieties north and south:
North poles repel each other and south poles repel each other. North and south poles attract each other. The fridge magnet works by “inducing” the opposite pole in the metal door on the fridge — but that’s a little advanced for our purposes so I won’t explain that here.
The compass needle is a magnet and the planet earth is also a giant magnet! There is a magnetic force from the poles of the earth on the poles (the two ends) of the compass needle, so that it aligns its south pole with the earth’s north pole and points north.
Electromagnetic waves are electric and magnetic forces which travel through space without any charges or magnets in them. Of course we need to have some charges and/or magnets somewhere to get the waves going — think of ripples on a pond when a stone is thrown in. We need the stone to get the wave going, but after that the wave just keeps going out from that spot on its own.
A water wave makes things floating on the surface move up and down. An electromagnetic wave makes charged things move up and down and it makes magnets turn back and forth, in an attempt to align their poles with the passing magnetic force or “magnetic field”.
One example of electromagnetic waves is the signal (wifi or otherwise) on your phone or device. These waves make the electrons in the atoms of the antenna shake — and that shaking is processed by the computer in your device and turned into data.
Another example is your microwave oven — here the shaking is converted into heat.
The purpose of this post is to underscore that another example is light itself! Light is an electromagnetic wave!
Radio waves, microwaves, wifi and mobile phone signals, light, X-rays, and gamma rays are all the same thing: waves of electric and magnetic forces.
What makes these waves different? Look again at the water ripples:
The distance between successive ripples is called the “wavelength”. It is simply the wavelength which makes the difference between gamma rays, X-rays, light, and radio waves. Green light is around 550 nanometers in wavelength. A nanometer is one millionth of a millimeter — so the wavelength of light is really, really short. The wifi signal on my 5 GHz home router is around 6 cm — much, much longer, but in every other way exactly the same as light.
So how is the wave connected to the ray? The wave travels at right angles to the crests of the wave or “wavefronts”. Below I have drawn a ray with the wave crests superposed — this is like looking down at a water wave:
Sometimes the wave nature of light isn’t important — this is when ray optics or “the ray approximation” can explain what is seen. Other times the wave nature is essential to understanding what is seen. Keep reading the blog to see some examples of when wave optics comes into play.