this is something that actually came out
of the comments on one of the previous
videos we did about bof fringant and
somebody wanted to know whether double
rainbows with the same kind of effect um
so I thought I'd talk about
rainbows so really to explain this I
need to do an experiment I need to show
you what a rainbow actually is so I have
here the sun and a raindrop not a scale
I have to say um and so hopefully with
these two fine tools we should be able
to demonstrate the physics of rainbows
to switch to lights
okay so if you kind of come down to my
level okay so if you work your way
around towards the right there's a kind
of a little you can see the little white
dot that sort of reflected off the back
of our giant raindrop there I can see it
and as you go around you'll see that it
kind of moves towards the right so it
kind of comes towards you and then at
some point suddenly it turns colors so
you'll see it Go blue green red and then
disappear
entirely and the point is you're
currently 42° away from the direction
that the light's shining in from when
you're at that
nice okay so what we've got is we've got
the light coming in and as you can see
as it passes through this surface so it
goes from one medium to another the
speed of light changes which has this
effect of changing the direction that
the light's traveling in um but the
amount by which it gets changed depends
on the wavelength of the light blue
light gets bent a little more than red
light so you can see that the Blue Light
reaches the back of the drop at a
slightly different point from the red
light
and then when they get reflected back
again and again there's another bit of
refraction when they come out the rain
drop again um and you can see that
basically the light the red light at
least in these extreme Rays ends up
traveling slightly at a slightly wider
angle than the blue light that's when
when you as you were panning around the
last Light you saw was the red light so
you see just normal reflection pretty
much all the way around to here and then
when you're at this last point with the
last angle that stuff can actually get
through the drop at all you first see
blue light then you see red light then
you see nothing further out at all of
course that's not what happens with the
rainbow with a rainbow what you have
show another picture is of course you
have lots and lots of drops which means
there's always one at the right exactly
the right angle to send the red light
towards you or to send the blue light
towards you and so actually what you end
up seeing is that kind of remember
there's this magic angle of around 42°
anything that's around
42° this angle here between the
direction the sun rays are coming from
and the direction you're looking from uh
you'll end up seeing either red light or
a little bit further out you'll see blue
light that actually means means that
rainbows actually are are round right
there's actually there's a circle of
points that are 42° away from from these
lines of the sun um but what actually
happens is that half of that circle is
typically below the Horizon so that you
see like half a circle of the of the
light and so you'll end up just seeing
the the classic kind of rainbow
semicircle if you have to be at this
magic 42° angle how come I can see a
rainbow and Johnny Smith 5 Mi away can
see a rainbow because he's looking at
different set of raindrops so he's still
looking 42 degrees away from the Sun but
from from where he's standing he sees a
completely different set of raindrops
that are all doing exactly the same
thing back towards you so when I see a
rainbow at my house and you see a
rainbow over at your house we're
actually seeing two different rainbow
you're seeing a different set of
raindrops that are causing the the
effect I mean obviously it's the same
Sun that's causing the effect and it's
the same physics but it's a different
set of raindrops reflecting it all back
to you and the other thing you noticed
is that when you went beyond so when you
were looking kind of closer to the Sun
or sort of closer to straight backwards
and forwards um so kind of when you're
looking at light just reflected off the
back of the Raindrop you saw basically
white light and it's only when you get
to these these extreme angles that you
actually see the the red and the blue
light and of course that's why if you
look at a picture of a rainbow you'll
always see that inside the rainbow is
always brighter than outside the rainbow
because inside the rainbow you're seeing
just mostly that whitish light being
reflected back it's the sum of all the
different colors being reflected back to
you so you see it as white light then
you reach this edge of it where you're
just about seeing just those last few
colors and then when you go to to to
wider angles still you see nothing at
all and that's why the outside of the
rainbow appears darker than the inside
of the rainbow
at the start I thought you were holding
out the promise of something about
double rainbows double rainbows we can
do too so Double Rainbows also again
classically understood going back at
least to as far as so dayart did this
experiment again in the 17th century
lots of people have had a go at this and
what they realized is so if we go back
to our original picture for just a
second so what was happening here is the
light comes in gets refracted reflected
once off the back of the Raindrop comes
back out refracted again in a double
rainbow what happens is the light takes
a slightly different path another
picture you can have the light coming in
gets refracted gets reflected once
reflected twice and then comes back out
again and comes out at a different angle
from if it had just undergone a single
reflection of the back this is another
way of making a rainbow at a different
angle so the first way still works you
can have things the light coming in and
getting reflected once and that'll give
us a rainbow at one angle or it can go
in and get reflected twice and then we
end up with a rainbow at two angles so
you end up with a picture like this so
we've still got the first rainbow here
where the light's coming in getting
reflected back to us just once off the
back of the raindrop
um and so we see the the colors spread
blue to red but there's a second way
that it can happen that it can actually
bounce twice around the Raindrop and
take this rather more complicated path
and what you can see is that actually
the rainbow ends up being reversed
because of that second reflection uh
where the red light was originally on
the outside if you look you at a second
rainbow you'll see that actually the
color is inverted that the the original
Rainbow the inner rainbow goes from Blue
to Red the second rainbow goes from red
to blue and in fact all the effects are
reversed so as we were talking about
before you see a brighter light down
here here then you see a normal rainbow
for the other rainbow everything's
reversed so you see the second rainbow
and then you see brighter light further
away and so the famously you see this
thing called Alexander's band which is
the dark region in between where there's
no white light gets reflected at all CU
in the first rainbow it's all ending up
down here and from the second rainbow
it's all ending up up there it seems to
me we should see a double rainbow every
time and yet we only occasionally see a
double rainbow it really is because that
so what happens is that in the Raindrop
most of the light just goes straight
through only a little bit of it gets
reflected and so actually it's a very
inefficient process most of the light
just travels straight through the
Raindrop a little bit of it gets
reflected of course if you want to
reflect it twice that means a little bit
gets reflected and then only a little
bit of the little bit gets reflected and
so the second rainbow is always much
fainter than the first so it is actually
always there and in fact very
occasionally you can even see a third
rainbow which is even fainter still so
these other Rainbows are always still
there but they're always much fainter so
if you can only just see the primary
rainbow you're not going to see the
secondary rainbow at all and it's always
when you have a really stonkingly bright
primary rainbow that you end up seeing
the secondary one people often ask this
question you know don't don't you lose
the magic of something by actually
understanding it you know by actually
understanding where a rainbow comes from
isn't it not beautiful anymore because
suddenly you've explained it and it's
all very simple and trivial and of
course it doesn't right I mean you know
I understand the physics of rainbows but
I could still look go out and look at
one and say wow that's pretty so it
really did you know doesn't have that
negative effect at all it's just it just
adds to the experience of of seeing a
rainbow then not only does it look
beautiful but you actually understand
why it looks beautiful too