You know, by and large, I like little kids. They’re insatiably curious,
which makes all of them potential future scientists.
They ask great questions. Probably the most common question they ask is “why is the sky blue?”
Given that many adults don’t know the answer, I think it’s a great topic for today’s video.
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Answering the question “why is the sky blue?” is actually super simple.
It’s because the sun is out and it’s not cloudy. Otherwise, the
sky would be black or white or gray. Duh. Man, that was a short video.
But seriously, why is the sky blue on a sunny day? If you ask random people,
they might say any number of things. They might say that it’s because the sky reflects the ocean.
They might say that it’s because air is blue, because, it, like, glows blue or something.
But we know those aren’t true. For example, air is still blue over vast deserts located far from
water, like the Sahara Desert. And you have air all around you. It’s obviously not glowing
blue. Besides, if the sun isn’t up, air isn’t blue. So, the sun has something to do with it.
Of course, we know the Sun itself isn’t blue. So, what's the deal?
It turns out that the sky is blue because of three specific factors. They are: the color
of light from the Sun, details of air molecules, and finally the specifics of the human eye. Let’s
see how these factors work together to answer one of childhood’s first scientific questions.
Let’s start with the Sun. If you look at the Sun- and you should definitely not
look directly at it- it’s very bad for your eyes- the Sun is basically white.
Now this might seem weird to hear, but from a physics point of view, there is
no color that is white. White is actually a mix of all of the colors of the rainbow.
This was discovered by no less a luminary of science than British physicist Sir Isaac Newton.
And this is in spite of what you might have heard
about another British phenom being responsible for it...
Nope, nope- it was definitely Isaac Newton. He found that if you sent white
light through a prism, you could break it up into the colors of the rainbow. He
also found that you could combine those colors and the result is white light.
So that’s what’s going on with the sun. It gives off all wavelengths of light, including
both visible, ultraviolet, and infrared light, the last two of which are invisible to the human eye.
In fact, only about 40 percent or so of sunlight is something we can see.
Here, I have overlaid the rainbow of visible light, and you can see that a
lot of it is in the infrared range. Infrared is heat and that’s why sunlight is warm.
Let’s zoom in to highlight the visible spectrum. The rainbow is familiar and we
see that the wavelength of light ranges from a short 400 nanometers to a long 700 nanometers.
So why does the sky appear blue? It’s because of how air molecules interact with light.
You see that air molecules scatter light- all colors of light- but they scatter
short wavelength light more. They scatter violet light- which is the shortest wavelength light
we can see- the most. They scatter blue a little bit less, green even a little less,
and so on. This picture here shows you kind of how it goes.
So the big idea is that the sun emits light of all wavelengths – red, orange, yellow, green,
blue, indigo, and violet- the whole ROYGBIV thing. This light, which looks white to us,
passes through the atmosphere and some of it scatters.
The blue light scatters more than the red, and so that means that the sky is blue.
The blue light we’re seeing is the light scattered in the atmosphere. That’s all there is to it.
Now, if you were listening closely, you probably have a question.
I said that air scatters short wavelengths, with violet being scattered the most. So why
isn’t the sky violet or some shade of purple? That’s where the human eye comes into play.
The human eye sees light and color by the use of cells called rods and cones. It’s all squishy
biology stuff. Cones are the cells that help us see color. And there are three kinds of cones.
Two of them are most sensitive in the green region, although one of them has
some ability to see red light. The other cone is sensitive to blue. And that’s the problem.
The cones that see blue are pretty good at seeing blue, but they don’t do so well with violet.
So, technically, the sky should be more on the purple side if we had perfect eyes. But we don’t.
Since we don’t see violet well, that’s why the sky is blue. And, of course,
because there are no clouds out.
This explanation also answers why clouds are white. Clouds are made of water droplets,
which are bigger than air molecules. Water droplets don’t scatter specific
colors in the way that air does. And, if all colors are scattered more or less equally,
then the result is that the colors combine to be white. That’s what white is, after all.
And gray clouds are just white clouds that are thicker and with more water.
They scatter so much light that some of it gets scattered back into space or trapped in the cloud.
From the perspective of someone on the ground, it’s just that the clouds are blocking the sun.
This scattering idea, which is called Rayleigh scattering by the way, is named after Lord
Rayleigh, who published the first papers explaining this phenomenon back in 1871.
The first observation was two years earlier by a scientist by the name of John Tyndall,
who first realized that blue light was scattered more than the other colors. But he didn’t publish
and Rayleigh did and, in science, publication is what matters. So Rayleigh scattering it is.
Rayleigh scattering also explains why sunrises and sunsets can be so spectacularly red. When the
sun is on the horizon, light has to pass through a lot of air before it gets to your eyes. That means
that even more blue light than usual is scattered, leaving preferentially the reds and oranges for
you to see. And if there’s a lot of smoke or dust in the air, these particles are also often small
enough to preferentially scatter blue light and that’s when you get the most amazing sunsets.
So that’s it. The answer to a toddler’s question involves such things as the fact
that the sun emits light in all colors, the air scatters blue and violet light the most,
and the human eye has trouble seeing violet.
So, the sky is blue. Now you’re ready for the next precocious two-year-old you meet.
Okay, so that was pretty cool. Hopefully you enjoyed the video, in which case,
I hope you click the like button below and share it on social media. Even though I often
talk about particle physics and the cosmos, it’s good to return to the basics and remember what
it’s like to view the world through the eyes of a child- the eyes of a person who hasn’t achieved
the most crucial insight of science, which is, of course, that physics is everything.
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