Consider the photon. They are the basic unit of light. They come in all sorts of energy levels. Collectively, photons make up the electromagnetic spectrum.
Let's follow the life of a photon from the core of the sun where it is created to its ultimate demise.
Simply put, the sun is a giant explosion. It is a nuclear fusion bomb that has been going off for close to 4.5 billion years and will continue for another 4.5 billion years before running out of fuel.
Fusion occurs deep in the heart of the sun. There, the combination of pressure and heat force the nuclei of hydrogen atoms together to fuse into deuterium nuclei which fuse to give helium. It is referred to as the pp-chain and yields 26.2 MeV of energy.
This energy is in the form of high-energy photons. Gamma rays or even shorter wavelengths of light are released. Fortunately, they do not travel directly from the core of the sun to you and I. If they did, we would never have evolved due to radiation damage.
In the core of the sun, the free distance or mean free path that a photon can travel before encountering another particle is very small - no more than one tenth of a millimeter. The photon is emitted and absorbed repeated.
Yes, it is traveling at the speed of light or 300,000 km/second and the distances are short but in order to move outwards from the core, a photon has to encounter millions upon millions of atomic and sub-atomic particles.
Imagine being in a crowded room and trying to get from one side to the other. Not the easiest thing to do, but it can be done. However, add in the complication that everyone you meet wants you to stop and chat. Suddenly a difficult task gets very much harder.
This is the photon's journey out of the sun's core. Further, unlike us with our intention of getting from one side of a room to the other, the photon's direction is random. Each collision sends it off in a new direction.
It is a bit like trying to navigate that crowded room with a blindfold on and each person we encounter giving us a spin. If walking through the crowded room is difficult, now it would be almost impossible.
Yet, it happens. The photon slowly moves from the core to the surface of the sun. Along the way, it loses energy shifting from harsh gamma rays through X-rays and into the ultraviolet and visible region of the electromagnetic spectrum.
The effective temperature of the sun's surface is about 5,700 K or "white hot" and it is from this region of the sun our photon finally escapes.
How long did it take to get from the core to the surface?
That is an interesting question as it is predicated on assumptions about the structure of the sun and the way that photons and matter interact. However, using a reasonable model of the solar interior, most answers would tell you the photon is between 10,000 and 170,000 years old.
Even moving at the speed of light, the number of collisions - the number of times the photon is absorbed and re-emitted - is so large that it takes a long time to travel a short distance.
However, once our photon is free of the sun, it only takes eight minutes and 20 seconds to reach Earth. Hardly any time at all. Indeed, the trip through our atmosphere is over in less than one thousandth of a second.
That is unless the photon is in the ultraviolet portion of the spectrum. If that is the case, then it is far more likely the photon will be absorbed in the upper atmosphere by collision with an ozone molecule. The ozone layer is such an effective screen that it captures 34,999 out of every 35,000 ultraviolet photons.
Or if the photon is at the other end of the electromagnetic spectrum - in the infrared region - it will also not make it through the atmosphere. Actual heat from the sun is trapped by gases such as carbon dioxide, methane and water vapour in the atmosphere. This is part of the greenhouse phenomenon.
It is really only photons in the visible region of the spectrum that are able to penetrate to the surface of the planet. The heat we feel from the sun is a consequence of a visible photon striking our skin and exciting molecules there. Some of its energy is dissipated as heat and these photons are re-radiated back into space or into our bodies.
Some of the energy is absorbed to generate photochemical reactions. Indeed, it is just such a fate - the death of a 10,000- to 170,000-year-old photon in our retina - that allows us to see the sun and everything around us.
