the lab
Milky Skies
Ever wonder why the sky is blue but sunrises and sunsets are red and orange? The Sun
itself never changes colour, so it must have something to do with the atmosphere. This
simple kitchen experiment will have you seeing red, blue, and the answer all at once.
What you’ll need:
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1 clear glass WIDE bowl, at least 2 L capacity (an empty aquarium would also work)
5 mL (1 tsp) measuring spoon
water
milk
flashlight or bicycle light (incandescent or white LED)
What to do:
Fill the bowl with water. Pour in 10 mL (2 tsp) of milk for
every 1 L of water – this will give you a 1% concentration.
Stir the milk so it’s uniform.
What colour is it?
Shine the light horizontally through the bowl. Look at
the colour of the light right next to the flashlight.
Then look at the flashlight THROUGH the bowl from
the opposite side. Feel free to try this in a dark
room if you want to see the colours extra brilliantly.
Do you see blue and red?
What’s going on?
Milk isn’t a pure liquid. It’s called a colloid, which is a mixture of suspended
microscopic balls, called colloidal particles, in a solid, liquid, or gas. In the case of milk,
these particles are blobs of protein and possibly fat, suspended in water. These balls
reflect light in all directions, something physicists call scattering. The colour
scattered the most is related to the size (and shape) of the colloidal particles. In the case
of milk the colloidal particles are of a size that blue is scattered the most. This is why
diluted milk in water has a hazy blue appearance.
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the lab
The sky also contains suspended colloidal particles – dust and molecules. The sky is blue
because the white light from the Sun has its blue light scattered around the most.
If the light passes through more of a colloid, the blue will scatter out completely,
leaving longer wavelengths of light like yellow, orange, and red. You can see this in the
bowl on the opposite side of the light source. Even a bluish LED flashlight will become
orange when shining through enough water and milk. In the atmosphere, the position of
the Sun at sunrise or sunset compared to noon means the light passes through
more atmosphere, so the light we see from the ground is orange or red.
NOON
SUNSET
This is also why as you go up in a plane, the sky gets bluer.
Smoke is another type of colloid, and often contains particles of a size that both absorb
blue and scatter red.
So on hazy days or
near a large fire, the
Sun can appear red.
After large volcanic
eruptions like Mt.
Pinatubo in 1991, so
much ash is thrown
into the atmosphere
that sunrises and sunsets around the world appear more red for months or years.
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