Re: why do we get red sunsets & how can I show this as an experiment

Hello, Kurtz, and thanks for your question. To answer it, I’m just going 
to say a little about visible light for openers.
Visible light is a small range of wavelengths in the electromagnetic 
spectrum – which goes from gamma rays and x-rays at the short wavelength 
end to radio and radar at the long end. Our eyes see only a tiny fraction, 
from about 400 nanometres (a nanometre is one billionth of a metre) to 
about 700nm, and humans have three separate types of receptors for colour 
vision called cones. The signals from these are processed in our brains to 
give us our perceptions of colour. We see red at the longer wavelength 
end – near 700nm and we see blue at the short end near 400nm. 
So, we are seeing long wavelengths in sunsets and sunrises, and short 
wavelengths when we look up at the blue sky. The reason for this is that 
the white light from the sun has to travel through the Earth’s atmosphere 
to reach our eyes. At noon, with the sun at its highest, the path through 
the atmosphere is shortest. At sunset, though, we are looking at the sun 
through a much longer path of atmosphere. The atmosphere is full of 
particles – water droplets, dust etc. These scatter light – but small 
particles scatter short wavelengths much more than they do long. This 
means that as the sun sinks in the sky, and the path containing these 
particles gets longer and longer, less blue light gets through as it is 
scattered more. This leaves the normally white light filtered to a red or 
orangey colour. Meanwhile, further west, where the sun is still high in 
the sky, people who look skywards see all this blue light which is being 
scattered by the atmosphere. Hence blue sky and red sunsets are due to the 
same effect – differential scattering of light of different wavelengths.

To demonstrate this with household materials, here are a couple of 
suggestions:

Milk is a suspension of fat particles in water, and appears bluey-white 
when diluted for the same light scattering reasons. If you can find a nice 
rectangular glass tank (fish tank) – and the longer the better, fill it 
with water and add a couple of teaspoons of milk. Not too much – just 
enough to make it go a little bit cloudy. If you have a good torch – a 
Maglite is good for this, shine it through the tank from one end to the 
other. You shoud be able to observe the light emerging at the far end 
being much redder after adding the milk than before, and equally the light 
scattered out of the sides of tank will be blueish.

Another trick to show the effect with a Maglight is with “clear” 
gluesticks. They are not completely clear, and contain small opaque 
particles which scatter light. If you have several glue sticks try shining 
the light lengthways through one, two, three and so on by taping them end 
to end. You should see the end where the light emerges get redder and 
redder as you add more sticks.

In the chemlab., you can make colloidal suspensions to show the effect 
even better. Your teacher can do this with sodium thiosulphate and 
sulphuric acid. Adding the acid to a solution of thiosulphate causes 
precipitation of very fine colloidal particles of sulphur which makes for 
very good light scattering.

Have fun experimenting, and thanks for your question.