OK.
So all that glitters is gold.
That's bad singing.
Glitter-- all that glitters is gold.
Gold, gold.
So you maybe you've heard of that, that little saying,
all that glitters is gold.
Yeah-- no.
Gold glitters, but all that glitters
is not necessarily gold.
All that glitters has no or a small band gap.
Doesn't really roll off the tongue the same way.
You can see why they didn't put it in that song
I was trying to sing earlier.
But that's a little more accurate.
All that glitters has no, or a small band gap.
Well, what does glitter mean?
I mean, what does glitter mean?
Glitter means that it's shiny, right?
It's shiny.
What does shiny mean?
Shiny means that we've got most of the incident light--
that's the light that's hitting the surface-- is reflected,
and emits a little light.
Actually, I should write light.
But with light, we got to be so specific with this.
It's not enough to say, all that glitters is gold.
We've got to talk about the band gap.
Now, Glitters is not good enough.
We've got to be specific.
Incident light-- you know, again, we're
talking about light in the visible spectrum.
This is visible spectrum.
Well, we can expand on that actually a little bit later,
but let's look at this.
What does this mean?
And really-- OK, I'm being kind of silly here.
I'm a funny guy-- well, I don't know if I'm funny,
but I'm weird.
You can think what you want, but we
want to consider why a metal looks the way it does.
Why is a metal shiny?
That's really what I'm trying to get at.
So this is our band structure here,
and we have these filled states in a metal,
adjacent to unfilled states.
And so what happens is when a photon of light-- a photon
having energy e equals hc over lambda
is incident on a metal-- there's our metal.
That photon energy is enough to excite an electron up
from one of these filled states to a vacant, unfilled state.
So this photon here is going to cause excitation, or promotion,
it's sometimes called.
That means that the photon there is absorbed,
but this is only a temporary excitation.
After a period of time-- that's temporary-- after a period
of time, that electron will drop back down.
And what happens when it drops back down?
It re-emits a photon of light.
So because metals have no band gap,
visible light can promote electrons
into these unfilled states, absorbing all the photon
energy.
And then subsequent transitions back down to the lower energy
states for the electrons result in this re-emission.
Emission, there you go.
I spelled it right that time-- re-emission.
And that's why metals appear shiny,
because they're re-emitting essentially
across all wavelengths, the whole spectrum.
Now, you might wonder, OK, why is it then
that gold is kind of yellow?
Gold versus maybe aluminium, which appears more silvery.
And what's the difference?
Well, there's some absorption that-- there's
some absorption of-- and without re-emission of some
of the short wavelengths.
And so we get a slightly different colour,
but it's still re-emitting, and so it still appears shiny.
And so that's a breakdown of that simple statement,
all that glitters is gold.
And it gives us a good first look at the colours and optical
properties of matter.