so today what I'd like to talk about is
one portion of the electromagnetic
spectrum and hopefully we'll explore the
rest of this vast regime in future
videos lights so what we have here is
the optical portion of the
electromagnetic spectrum this is of
course the rainbow that's familiar to
everyone our colors going from red to
yellow to green to blue indigo violet
and as we progress through the rainbow
the energy is associated with the
photons of light get higher and higher
and the wavelengths get smaller and
smaller light is simply ripples in the
electromagnetic field the wavelength or
the size of those ripples governs the
energy and that ends the color of the
resulting light but what we see here in
the optical regime what our eyes are
sensitive to is only a tiny fraction of
the story
in fact the electromagnetic spectrum
stretches all the way from x-rays to
radio covering in between the
ultraviolet the infrared gamma rays all
sorts of other different regimes that we
may not necessarily think of as being
light but certainly are and certainly
are of use to us as astronomers but by
focusing in on only one portion of the
spectrum it's as if we're limiting
ourselves to borrow an analogy from
another astronomer David Hale found it's
as if we're only hearing one section of
an orchestra and listening to the
symphony
so as astronomers we like to exploit
multi-wavelength astronomy we like to
hear the whole Orchestra in play but by
listening to the various different
instruments by isolating the various
different wavelengths we can learn
different things about the objects that
we're looking at so the part of the
spectrum we want to talk about today is
actually the invisible bit off the end
of here so as we go past the blue and
the indigo violet we get to the
ultraviolet we get to wavelengths that
are too small for our eyes to be able to
see although some people with a medical
condition called a fakey oh where
they're missing a lens in their eye
actually are able to see wavelengths
down in this regime and see in the ultra
toilet lights thanks thankful so the Sun
of course radiates at all wavelengths
but the amount of light that gets down
to us here on the ground varies
according to wavelength and that's a
good thing because although most of the
light in the optical regime from the Sun
makes it all the way down to the ground
and we can see it with our eyes the
ultraviolet light is largely blocked by
the atmosphere and that's a good thing
for us because the energetic photons in
the ultraviolet are bad for our health
they cause sunburn they cause cancer and
we want that layer of the atmosphere
covering us and protecting us from those
energetic rays so that's not to say that
UV light is entirely bad in fact we need
some of it we need UV photons to create
vitamin D in our skin that's the only we
don't get vitamin D from our food so we
need sonics some sun exposure in order
to be healthy UV light has lots of other
practical applications it can be used in
medical diagnostics it can be used to
monitor hygiene you may have seen in
crime dramas UV black lights being used
to see splatters of blood or bodily
fluids and crime scenes we can also have
a lot of fun with it
because there's lots of ordinary
household objects that actually glow
when put under UV light this is called
blacklight oh yeah well you're seeing
you're seeing light coming out in the
visible because the the lamp is not
perfectly emitting in the UV but it does
have some filters that blocks out most
of the visible light but we can actually
make that ultraviolet light visible
using a process called fluorescence okay
let's poke thanks mate
so many ordinary objects undergo process
called fluorescence they absorb this
invisible light and emit it at
wavelengths that we can actually see and
you can actually have a lot of fun with
this so historically for a long time
physicists have known that the substance
quinine is it quinine or quinine
I don't actually know for a long time
physicists have known that the substance
called coining fluoresce is under UV
light and of course quinine is a very
important ingredient in tonic water
which is very important ingredient in
gin and tonics so I have a bottle of
tonic water here so we'll stick it under
the UV light here we go I'll put the
tonic water under the UV light and if we
turn off the lights we get a terrific
glow and what's happening here is that
the energetic photons from the
ultraviolet are being absorbed by the
molecules of quinine and electrons in
those molecules are being excited to a
higher energy level once they're there
they start losing energy again first
they lose it a bit through vibrational
energy and that's possibly really
emitted as heat and so by the time they
fall back to their ground state they
don't have as much energy to get rid of
and so the energy they emit is lower
energy than the energy they absorbed and
that means the wavelength is longer so
while they've absorbed ultraviolet light
they emit light in the optical and in
this case we get this lovely
bluey green glow lights so of course you
can have some fun with this because you
can start making things with tonic water
so last night
I whipped up a batch of lime green jelly
or jello for our North American viewers
and we can see again if we put that
under the blacklight light so on the
Left we have jelly made with the tonic
water and on the right we have jelly
made with just ordinary water and you
can see the difference the the tonic
water continues to fluoresce even though
it's in the jelly light all right
but coin is not the only substance that
fluoresce is under UV light that you can
find in your kitchen
in fact olive oil is another good thing
to have a look at and so what I've done
here is I've taken some olive oil and
you can see it's quite a green color and
that's because I've infused it with some
mint which is going to make this effect
even more spectacular and so you note
it's the same sort of greeny colour as
the jelly is but when we put it under
the blacklight we'll see something quite
different
so lights so now you can see that
instead of glowing greeny blue it's
growing this beautiful shade of red and
the reason that's doing that is because
the chlorophyll in the olive oil and in
the mint that I've added and so again
we're seeing the same phenomenon we're
seeing fluorescence energetic
ultraviolet photons being absorbed being
readmitted at lower energies at longer
wavelengths in this case the wavelength
is long enough that it comes out as red
so I discovered that particular secret
thanks to our lab guru Paul who it told
me the story of going through a
supermarket with a UV blacklight to find
the the freshest olive oil because this
effect doesn't last very long you can
have even more fun and games because you
can buy commercial paint and skin
pigments that allow you to do fun
effects under the blacklight so again
got some face paints here if we turn the
lights off you can see the fantastic
colors so put together these face paints
and glow-in-the-dark jelly shots and you
can have a really good party of course
you can get marker pens that allow you
to write invisible messages that can
only be seen under under black light as
well so we had some fun playing with
those but life so this is all fun and
games but to an astronomer UV light is
really important because in the universe
ultraviolet light is generated by hot
young stars and other extreme objects
like supermassive black holes
so we use ultraviolet telescopes which
we have to position above the Earth's
atmosphere because otherwise none of the
light is going to get to us and we use
these satellites to image the universe
and collect those ultraviolet photons
and that can give us a really different
picture of what the universe looks like
in my own research I mostly work on
clusters of galaxies which are made up
of collections of stars that are old and
red and passed sort of the active phase
of their life so there's not a lot of
ultraviolet flux coming from those
galaxies but when I look at the same
field using an ultraviolet telescope
like the Galax telescope my galaxies
disappear they become invisible and I
can't see
but what I do see is where there are
sites of intense star formation
where massive hot stars many times the
mass of our own Sun are being born
living for only a few million years
before dying possibly in a supernova
explosion leaving behind a black hole