It’s the quintessential question asked by toddlers, and I love the answer.
To get to the answer we need to lay down some foundation.
Number 1: the white light coming from the Sun is made up of all colours. Isaac Newton made a complete study of this, going so far as to identify seven individual colours in the spectrum created by light passing through a prism: red, orange, yellow, green, blue, indigo, violet. Truth be told, the colour indigo didn’t exist before Newton. He made it up as a colour between blue and violet because, among other reasons, seven colours was theologically more pleasing than six.
Number 2: the Earth’s atmosphere is not one hundred percent gas. It also contains tiny particle of dust, particles from smoke, microscopic droplets of water and other liquids.
When light is passing through our atmosphere, the parts of the light spectrum at the blue end are more likely to encounter a teeny particle and be deflected off-course. Light from the other parts of the spectrum are more likely to pass through unimpeded.
You observe this effect when the Sun is close to the horizon. The light from the Sun has had to pass through a large amount of the Earth’s atmosphere, so contains very little blue light and is mainly made up of oranges, reds, and yellows; the colours of sunset.
The blue parts get seen by people on Earth where the Sun is higher in the sky. This light is raining down on them from all directions, so it is as if the whole sky is glowing blue.
The effect is called ‘scattering’, and was first studied by John William Strutt, the 3rd Baron Rayleigh, otherwise known as Lord Rayleigh.
You can create the effect yourself by using a glass of water, a strong torch, and some milk. (I prefer using coffee whitener, but milk will do.)
In a dark room, shine the torch through the glass of water so that the beam is pointed to a white surface. Put in a drop of milk (or few grains of whitener). Less is more with this demonstration, so only at the very slightest amount milk. Mix the water and look carefully at the light glowing from the glass, and the light hitting the wall.
The milk (and the whitener) does not dissolve in water. Instead very small particles become suspended in the water. As you add more particles, then more of the colour spectrum has a chance of being deflected off-course.
As you add more milk, the glass should take on a blue-ish hue, while the beam on the wall will gradually become darker while cycling through oranges to reds. Eventually, there are so many particles in the water that every part of the spectrum is likely to be deflected, and the glass will be glowing white while the beam no longer appears on the wall.
This Rough Science was initially suggested by @sorrel_smith and her daughter. But how appropriate is the answer for someone under five?
No answer lives in a vacuum. Knowledge is layered and knitted. So if your toddler says “why is the sky blue”, do you tell them about the visible light spectrum, wavelengths of light, and Reighliegh scattering?
I would prefer to avoid the following:
“It’s paint. Dulux range, sky blue.”
Instead I would try to give them a near-truth.
“The light from the Sun makes the whole sky glow blue.”
But why blue?
“Because of Earth’s air. If it was different air, it could be a different colour.”
It’s breadcrumbs leading the way to the whole answer. Also, it helps to answer other questions.
“When the Sun goes down, the glow goes away, and we can see the stars.”
True knowledge often doesn’t fit into a sound-bite. If understanding could be made in just one sentence, there would be no need for homework.