What is solar energy - The Sun. Photo courtesy of NASA
The sun is a fairly boring, main-sequence star. It is about 4.5 billion years old and will exist at least that long again.
Solar energy comes predominantly from a process called nuclear fusion, whereby small atoms like hydrogen and helim combine to form larger elements, and emit energy in the process.
A main-sequence star like the Sun is a stable combination of gravitational forces, caused by its mass holding it together and gas pressure, caused by its density, keeping it from collapsing.
How much solar energy reaches Earth?
This is a classic physics question. The "luminosity" of the Sun, the total amount of power it emits, is 3.8 x 1026 W. Just to be clear, that's about 4 with 26 zeroes after it.
This energy is being emitted in all directions, spherically. Imagine a sphere of light moving outwards from the Sun. As the sphere moves outwards, the amount of energy per square metre goes down, as the same amount of energy contained on the surface of this bubble covers a larger area.
Imagine this imaginary sphere is now at the Earth, so the radius of this bubble of energy is the distance from the Sun to the Earth. Now we can work out how much energy the Earth receives per square metre at the top of the atmosphere:
First, we need to work out the surface area of this bubble. The surface area of a sphere with radius R is 4πR2 (that's a very poor symbol for pi, but a pi it is nonetheless!), measured in metres squared.
If R is the Sun-Earth distance ( 1.5 x 1011m ), we can find that the amount of energy per square metre at the top of the atmosphere is the luminosity (measured in Watts) divided by the surface area of this sphere:
3.8 x 1026 / 4π x 1.5 x 1011 x 1.5 x 1011 = about 1344 Watts per square metre.
That means a one square metre solar panel in orbit would receive 1344 W, enough to power a house. Unfortunately, we can't extract 100% of this energy from solar panels. We also have to remember that the atmosphere removes much of this before it reaches ground level, which we can then use.
What is solar energy made of?
There is a concept in physics called "black body radiation", which gives the ideal distribution of energy across the electromagnetic spectrum of a hot body. We can approximate the Sun as being a black body with a known temperature.
We find that the actual solar spectrum (distribution of energy) fits this theory quite well, as the diagram shows (the black line is the theoretical black body).
The wavelength of the light corresponds to its energy and so its position in the electromagnetic spectrum.
All of these are present in sunlight, but we our eyes have evolved such that we only see the "visible" part.
Infrared is classically associated with heat transfer (for example you radiate infrared as you are warm, which is how thermal cameras can see you in the dark, but you do not emit visible or ultraviolet light because you are not hot enough).
Ultraviolet light, which has a short wavelength, is absorbed in the atmosphere by ozone, which is useful as it is damaging to skin and eyes.
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