How Rainbows with NO COLOR Are Possible

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 like-- 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 I 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.

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 plane 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 spider webs.

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.

OK, 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 going to 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-- that's-- whatever.

[LAUGHTER] OK. Now picture a beam of sunlight entering a water droplet, which we're going to 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 going to bend again when it goes from water to air.

So it's going to 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 of different angles.

And you can get that 42 degree 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 going to 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 going to 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 mists 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 kind of hanging out, can be up to 1,000 times smaller than raindrops falling from the sky.

So raindrops are typically on the order of one millimeter.

And mist droplets can be as small as 0.001 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 glories, so we're not going to get into that.

But I've linked to the source in the description if you're morbidly curious.

Stepping back a bit to those 22 degree halos, there's a related phenomenon called sun dogs that I usually see as 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 passing 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, they're 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 defracts 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 spider webs that a follower sent me?

It said the spiders have "gone ham on the lawn," which made me laugh.

I initially thought it was the light refracting through the spider webs, 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.

Now this has been Physics Girl rainbow-y edition with Dianna.

Thank you so much for watching.

Send me, tweet me all of your rainbow pics.

I love seeing them, especially if it's something weird and interesting.

Thanks for watching and happy physics-ing.