- Hey, I'm Dianna and you're watching Physics Girl.
So a little while back
I was walking around the bay in San Diego
when the view struck me.
I saw this picture of something that looked
kind of like a white rainbow.
I thought at first it was maybe a cloud
but that's a weird placement and shape for a cloud
coming off the bay.
It looked like part of a rainbow, not a great view.
But it took a picture of it,
this is the best I could capture,
and then I forgot about it until months later
I saw some articles on the internet
pop-up about white rainbows.
These ones are much more defined than mine
but clearly a thing.
These are beautiful.
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I wanted to know what these white rainbows are,
and it started me thinking about
all the different strange rainbow phenomena
that I've seen out there
like the rainbow circle around a plain shadow,
the little rainbow gems in the sky
a few angular degrees away from the sun,
circular halo rainbows,
and perhaps the weirdest of all this one wasn't mine
but someone sent this to me,
rainbows on a lawn covered in spiderwebs.
Let's find out why
all these different kinds of rainbows form,
and why they're so much more unusual than regular rainbows.
First of all how does a regular rainbow form?
Well, it starts with refraction.
We can't do rainbows without refraction.
Okay, there's this weird thing that light does
when it goes from air to water, it bends.
This is called refraction. Yay! Love that word.
And it happens when light goes from air to glass,
glass to water,
anytime light passes the boundary
between one medium and another at an angle
as long as they have a different index of refraction,
which is a number that describes how fast
light moves through the material,
because light doesn't go at light speed through materials.
It only goes at light speed through a vacuum.
Light goes slower than the speed of light through materials.
Maybe you knew now you do.
So the index of refraction describes
how fast light goes in the material,
and it also describes how much light is gonna bend
when it goes from a material with one index
to a material with another index of refraction.
Got it? Cool.
But the index of refraction
which you now are very familiar with,
often depends on the color of light
going through the material
which is something they don't usually tell you
in physics class.
So for example violet light bends more as it enters water
than red light would bend.
So what happens is an incoming beam of white light
which is composed of all the visible colors of light
will get spread out and separated into a rainbow spectrum.
And this is just because violet and red light
travel at different speeds through materials.
That's kind of interesting,
that's, whatever.
Okay, now picture a beam of sunlight
entering a water droplet
which we're gonna pretend are spherical
even though we all know they're shaped like this
thanks to people who study water droplets in wind tunnels.
So beam of light hits the droplet
some of the light will refract into the droplet
then reflect off the back and refract,
it's gonna bend again when it goes from water to air,
so it's gonna refract back out
to travel down to your eye
typically making an angle of 42 degrees
between the incoming and outgoing light,
also known as the socially confident light.
Is that a bad joke?
Do you get it?
And that's how rainbows form.
Droplets of water in the air
are spreading out beams of light so that some droplets
are sending red light to you,
and some droplets are sending blue light, and violet light,
and yellow light from different places in the sky
because the different colors
are coming out at different angles.
And you can get that 42 degrees sweet spot
with a circle of droplets.
So if the ground weren't in the way
then rainbows would be a full circle
which is what some people get to see
when they're in airplanes or higher above the ground.
Which is not to be confused
with these interesting rainbow photos
which are like halos around the sun.
These are often called 22 degree halos
which is nice and descriptive
because if you take the whole view around you
as 360 degrees then the halo would be about
22 degrees of that.
And they're caused by millions and millions
of tiny ice crystals in the atmosphere.
The ice crystals have a different shape and structure
than water droplets.
So you're gonna get the light reflecting,
and refracting out at different angles
than if they were going through water droplets.
That's why it's at 22 degrees.
I actually took this picture. I love rainbows.
And this one was taken by my mom back home in Hawaii
which is a double halo rainbow.
I'm not gonna get into how double rainbows form
which is interesting but it was so YouTube 10 years ago.
But these halos are not to be confused with these rainbows
often seen around airplane shadows.
Yeah, I took these ones.
These are called glories or pilot glories
because they're typically seen from airplanes
looking down at clouds.
But sometimes from high mountains
looking down at mist or clouds.
And they're caused by light interacting
with tiny water droplets.
Like we're talking droplets
that are only the size of a few wavelengths of the light.
The water droplets that are in clouds and in mist
that are just kinda hanging out
can be up to 1000 times smaller than rain drops
falling from the sky.
So raindrops are typically on the order of one millimeter,
and mist droplets can be as small as .001 millimeters.
What is that? One micrometer?
Yeah, micrometers, tiny droplets.
So the droplets in clouds making these halos
are a lot smaller than water droplets,
and therefore interacting with the light
in a very different way.
In fact, they're interacting with the light
in a super complicated way.
One source I found describing how the halos formed cited,
geometric optics,
the complex angular method and catastrophe theory
as part of the explanation for the glory
so we're not gonna get into that.
But I've linked to this source in the description
if you're morbidly curious.
Stepping back a bit to those 22 degree halos
there's a related phenomenon called sundogs
that I usually see is what looks like
a little gem of rainbow in the sky.
But if you get a really good one
you'll see the 22 degree halo with bright rainbow spots
on either side.
And these are caused by light through hexagonal crystals
in the atmosphere.
We all know from snowflakes that water forms
these amazing hexagonal shapes when it's in its frozen form.
And so tiny hexagonal ice crystals
can form in the atmosphere and orient themselves
so the crystal face is almost completely flat
as they fall slowly through the atmosphere.
So you get more concentration of the 22 degree light
refracting on the horizontal edges.
So this all relies on the weird shape,
and properties of ice crystals.
The fact that we have gravity to pull them,
and make them flat.
The fact that we have an atmosphere to orient them,
and the place of the sun in the sky.
So these often form when the sun is low
close to the horizon, cool.
And if you get a really good sun dog
you get all these amazing extra optical patterns
all over the sky.
I've never seen anything like this. Someday.
And now how about the white rainbow?
Well, there's sometimes called fog bows
which is a really good hint because fog like mist and clouds
have really, really tiny water droplets.
And when water droplets get small enough
that they're close to the wavelength
of light passing through,
another optical phenomenon takes over called diffraction.
Diffraction is the slight bending of light around objects.
And when there are a bunch of tiny little particles
of water in the air the light diffracts,
and deviates from its path a bit,
and spreads out, smears out, the colors overlap,
and you get a white rainbow.
And finally what was that rainbow on the grass
with all the spiderwebs that a follower sent me,
and said the spiders would have gone ham on the lawn
which made me laugh.
I initially thought it was the light refracting
through the spiderwebs
but they probably don't have the right shape.
Ah, I didn't know until I read a little bit more closely
that there had been morning dew on the spider webs.
And so there were thousands of tiny little
substitute raindrops making a beautiful rainbow on the lawn.
Amazing, nature is crazy.
And this has been Physics Girl
rainbowy edition with Dianna.
Thank you so much for watching.
Send me, tweet me all of your rainbow picks
I love seeing them
especially if it's something weird and interesting.
Thanks for watching and happy physicsing.
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