Lesson 3b: More on the solar spectrum lines

Source: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Published: November 30, 2017


I was asked:

‘In the sun’s spectrum there are black lines at special wavelengths’
This threw me a bit, and so in the continuing paragraph I also got confused. The ‘spectrum’ is the range of light yes? I’m not sure why there are black lines in there. And so then are these black lines acting as blackbody? Are the black lines matter? or more electromagnetic waves?

So let me explore that a little more.

The spectrum is the rainbow – but extending beyond the visible. It’s the light spread out in different wavelengths. Above is the photo NASA took of the spectrum from the top of a mountain in Arizona, USA. Note that really it’s one line going from red to blue, (it’s not 2D – they just made it that way to fit on a page!). The colour you see is the spectrum from about 800 nm (top left) to about 400 nm (bottom right) – the visible part of the solar spectrum. (Note that the solar spectrum actually goes from about 200 nm in the UV to about 3000 nm in the infrared – but we can only see this little bit of it).

You also see black lines in the spectrum. These come from gases either in the outer part of the sun or in our own atmosphere that absorb light with particular wavelengths because that light has exactly the right energy (E = h nu) to make an electron jump from one orbit to another. Later the atom might release that energy going back down again – but, this is the crucial bit, it won’t do so in the same direction that the light was going in in the first place (and sometimes that will then cause another atom’s electron to jump up). So less light gets to us at those wavelengths than should – and we see black lines in our spectrum.

Lesson 3: My favourite equation

E=hnu picture

When I was 17 and doing A’level chemistry I learnt the equation E = h nu. This was the most exciting science lesson of my life. Seriously. I was only disappointed that chemistry rather than physics gave me that gift 😂

You see, I’ve always been fascinated by colour – the mixture of a prism for my tenth birthday and my dad being colourblind (a condition one of my two sons has inherited from the x-chromosome I got from my dad: the other son got the x-chromosome I got from my mum) meant that I really wanted to understand how “colour worked”. Now there are three bits to that – understanding our eyes (a detour I won’t go down now), understanding blackbodies – which explains white light – where lots of wavelengths are present, which we covered before, and understanding E = h nu which explains lights being coloured in and of themselves.

What this equation shows is how atoms interact with light. You’ll remember the simplified model of an atom with a central core and electrons in rings around it? Well, when an atom is “excited” the electrons can go up to a higher orbital. And when they fall back down to the ground state (the state where all the electrons are as close to the nucleus as possible while obeying the rules of how many electrons can be in each orbital) they release energy (based on the size of the jump) as light and the frequency of the light (nu) is proportional to the energy jump. So the bigger the energy of the jump, the higher the frequency of the light (more blue, less red).

You can see this yourself. Drop salt into a flame and the heat of the flame will excite the sodium atoms in salt to a higher energy state. As they fall back down they release yellow light. You might recognise that yellow light if you remember sodium street lights. In those electricity excited the sodium atoms. (Go on, try it!)

In the sun’s spectrum there are black lines at special wavelengths (called Fraunhofer lines). That is this process in reverse. The outer part of the sun is cooler than the inner part and absorbs light to make electrons jump up to higher orbitals. So the blackbody light from inside the sun (all wavelengths) loses light at the special E = h nu wavelengths. Of course those do re-emit those wavelengths as they drop back down, but here’s the crux: they re-emit in all directions including back towards the middle, so the amount of light coming towards us is lower.

This is how helium was discovered – they could match most Fraunhofer lines to lines they could get by putting elements into flames – but there were a set of lines they couldn’t match, so they proposed a new element – helium. (From Helios). Later they found helium on Earth. This is also how neon lights work – different jumps in neon give the different colours of neon light.

Now atomic lines are high energy so they tend to be in visible spectral bands. Later we’ll see that molecules have absorption/emission lines too. These are not from electrons jumping but from the molecule wobbling. Because whole atoms have to move the energy is much lower (heavier things don’t move as easily). So these lines are in the infrared.

But we’ll come back to that.

Lesson 1: Electromagnetic Radiation

EM Spectrum image

To understand climate change, we first need to understand light. (Personal aside: I got a prism for my tenth birthday and told everyone that one day I’d get a job splitting light into pretty colours – so of course I start here)

Light is electromagnetic waves that travel at the “speed of light”. The properties of light depend on the wavelength (how many times the electromagnetic field vibrates). Short wavelength light vibrates lots and the wavelength is small enough to get inside you and damage you – that’s “ionising radiation”: ultraviolet that damages your skin and x-rays and gamma rays that go inside.

Long wavelength light is radio and microwaves and the infrared. That can’t damage your molecules directly, but (and we’ll come back to this), some infrared and microwaves can make molecules vibrate which heats things up.

In the middle is visible light – the bit we can see. At 400 nm (nanometre – that means 0.000 000 400 m) wavelength we start to see blue light (if we don’t have cataracts) around 555 nm it looks quite green – and our eyes are most sensitive. At 800 nm we just about see a deep red (unless colour-blind and lacking red sensors).

Now it’s no coincidence that this is the bit of the electromagnetic spectrum that we see best. This is the peak of the sun’s spectrum – and all we see on Earth is visible electromagnetic radiation from the sun (or one of our artificial lights) that reflects from the Earth.

But the Earth itself does glow – just in wavelengths we don’t see. We call that the thermal infrared. In lesson two I’ll explain about blackbody radiation.