Why is the sky blue?

09 April 2006

Derek - Hello there and welcome this week to Downham Market High School. We've come here this week to the science lab to do some experiments. With me is a new recruit to the Naked Scientists who's going to be doing a few of these kitchen science features for us. Could you introduce yourself and tell us what you do please?

Sheena - My name's Sheena Elliott and I'm a PhD student studying physics at the University of Cambridge.

Derek - Excellent, so what is it we're going to be doing this week?

Sheena - Today we're going to try and do some blue sky experiments. We're actually going to be trying to simulate blue sky in the lab.

Derek - Ok, looking at blue sky. We've also got a couple of volunteers here who've very kindly come down to help us do it. So guys, can you tell us your names and what year you're in here please?

Matt - Hi I'm Matt and I'm in year 10.

Sarah - I'm Sarah and I'm in year 10.

Derek - Thanks guys. I've got to find out whether you're into science because obviously we're out here trying to promote science a bit. So Matt, do you like science?

Matt - Yeah a bit. It's alright.

Derek - And yourself Sarah?

Sarah - Not really.

Derek - So are you open to us persuading you it's actually really cool and fun?

Sarah - You could try!

Derek - Ok well we'll be trying to persuade you that science really is very cool. You at home of course can do this experiment as well. It's very very easy. The things you need at home are: some kind of vase or a transparent tall container; also you need a bit of milk; and a torch. That's basically it and now Sheena is going to tell us exactly what we do with all these things.

Sheena - Ok well first of we need to fill the vase or tall glass container to the top with water. We then just need to put in a few drops of milk. We really only need a little amount to put into it. Remember that if you put milk in, you can't take it out so just add a little at a time.

Derek - Well Matt and Sarah are here to try and help us with some of these preparations. We've already got a vase here but we also have some milk and we've got the glass filled up with water. So Sheena, can you instruct them what to do with the milk exactly?

Sheena - Yes. All you need to do is just take the milk, I've got a teaspoon here, and just add less than a teaspoon to the vase and stir it in.

Derek - So that's really a very small amount of milk guys so here we go. And so Sarah, what does it look like in there?

Sarah - Cloudy.

Derek - And so is that mixed enough or does it need to be more mixed?

Sheena - You might just need to stir it a little bit more so it's the same colour all the way through.

Derek - Yeah because it's kind of quite cloudy at that surface at the moment, so Sarah's just stirring it all the way round. And I think that's ok. What next?

Sheena - Now we're going to turn the lights out in the room, take the torch and shine it in through the side, and then look at the colours we observe in the vase.

Derek - What kind of different things, what different angles can you shine the torch from?

Sheena - Well if you start by shining the torch in from the side and then looking at right angles to the beam so we won't be looking directly into the beam. And the secondly, we can look through the vase directly into the beam with the vase in the way of the beam. And then finally, if we shine the beam from below the vase and look from above and see what that looks like.

Derek - Ok so you heard it there from Sheena. You've got to shine a torch into that slightly cloudy vase with the water and a bit of milk in it from a few different angles. Lots of things for you to do there and of course we will be doing that later here at Downham Market High School. Of course, Matt and Sarah are here ready to do it. So Matt, what do you think is going to happen?

Matt - Hopefully the effect we're looking for.

Derek - Well indeed! No more precise than that. Sarah, any idea?

Sarah - No I don't have a clue.

Derek - All sorts of answers there. Well we will certainly be finding out what happens later on. But people at home, you don't have to wait because you can do this right now if you've got these things at home. You can give us a call and tell us the result. We'll be back at Downham Market High School later on to find out what happens and explain how it all relates to things you see around you. Until then it's goodbye.


Derek - Hi there once again and welcome back to Downham Market High School where we've got this experiment all ready to go. Sheena's here ready with Matt and Sarah who helped us set up the experiment earlier. So we've got the vase, some water and a tiny bit of milk in there so it's ever so slightly cloudy. We've also got a torch ready to shine through it. But of course, we do need to do this in the dark, so the teacher here at Downham Market is very kindly manning the lights. So, lights please! We're in the dark now. Has anyone got the torch?

Sheena - Yes I have.

Derek - Good I'm glad about that. Ok, so hit the torch, and now what we need to do is for Matt and Sarah to shine some light through this vase and tell us what they see. What direction would you like them to do first them Sheena?

Sheena - So to begin if they just shine the torch into the vase from one side.

Derek - And now what can you see looking across?

Matt - A beam of light going across and the watery milk going blue where the light isn't.

Derek - Ok, so what's the next condition we can try ?

Sheena - So if we shine the light from the same direction but instead of looking at right angles to the beam, we look straight in at the beam from the other side of the vase.

Derek - So Sarah this time, do you want to try that?

Sarah - You can see through the glass. It looks the same.

Derek - Ok Sheena, what's the best way to see the effect we're looking for here?

Sheena - If you compare the colour of the torch beam when it's shone straight onto paper and then compare it to the colour when it's shone through the water and the milk onto paper, see what happens.

Derek - If you shine the beam straight onto paper, what do you see?

Sarah - Just the normal light.

Derek - And if we shine it through the milky water, what do you see then?

Sarah - It goes a lot more orange.

Derek - So Sheena, have we seen the right effect here?

Sheena - Yep. What we've seen is the effect we see with sunlight in the sky. Just to explain a little bit about light first. Light is made up of all different colours. You've got the whole spectrum from blue through to red. When the light shines onto the liquid, you've got little globules of milk. This is an emulsion, which is little globules of milk floating in water. This light comes in and it hits these globules of milk, but the different colours behave in different ways and this is called scattering. Blue light is strongly scattered. Originally when you saw that blue haze, the blue light scattered all over the vase. It was hitting these globules of milk and was being scattered in all directions and we could see this light being emitted from all parts of the water.

Derek - Just quickly, why is it that the different colours of light behave in those different ways? Why does blue scatter?

Sheena - It's a property of how much energy it's got. It's a different wavelength so it's got a shorter wavelength compared to the shorter light and it's also going to be an effect of how big the milk globules are.

Derek - Now, where do we see this effect all around us?

Sheena - So we see this effect every day as we look up into the sky. The sky is blue because the sunlight is shining down towards us but as it's shining towards us it's hitting all these air molecules and being scattered away. When we see a blue sky, we're just seeing the blue light that has been scattered in all directions. We then see it further away from the sun as it bounces into our eyes.

Derek - And so what about when we shone it through and saw that there was more of a red colour on the piece of paper when the torch light was actually going through the milky water? What's going on there?

Sheena - That's basically because we've removed the blue light. Like I said, light is made up of all these different colours, but when we remove the blue light, the white light no longer looks so light. That's'why we see the red because the blue has been removed. If you find that you're not seeing the effect if you've got a small vase or something, if you shine the light up from below so it's travelling through more water, then a lot more of the blue light will be scattered. The light will then appear much more red.

Derek - So of course this reminds us of something we see every day which is…

Sheena - The sun basically. We see a yellow sun because some of the blue light has been removed and scattered around into the sky. Also, when the sun sets, it's actually at an angle to the Earth and coming through a lot more of the atmosphere. This means that a lot more of the blue is taken away and that's why we see these lovely red sunsets.

Derek - Ok Matt, does that all make sense to you?

Matt - Yeah that makes perfect sense.

Derek - Good stuff. And Sarah, our mission here tonight was to convince you that science really is great fun. How have we done?

Sarah - You've done good. I think it's alright now.

Derek - Are you going to change your outlook completely?

Sarah - Maybe a little bit.

Derek - Well that's great. Thanks for doing this experiment with us and thanks to Sheena for setting it up of course, and to Downham Market High School for having us. We'll be back next week for more Kitchen Science that hopefully you can do at home. Until then, it's goodbye from us.


A lemonade Bottle or large glass

A large Jug  or a bowl and a funnel.


A little milk powder

A small powerful torch

A dark room


Fill the jug with water

Add the milk or milk powder - do this very slowly as you only need a tiny amount. 5-10 pinches will probably be sufficient

Turn the lights off

pour 1-2 cm of liquid into the bottom of the bottle.

Shine the torch up through the bottom of the bottle.

Look at the light coming straight through the bottle and the light coming out of the side, what happens to the colour?

Add some more liquid, and look thorough it again (repeat)

If nothing happens to the colour, you need to add a bit more milk powder, if the light doesn't get through the bottle, you need a bit less


The more milk the light goes straight through, the redder the light should look. So with no milk the light bulb will probably look white, then go through yellow, orange, and if you are lucky end up looking red.

If you look at the side of the light beam, you may notice a blueish tinge to the light coming out of the side near to the torch and the scattered light will look more and more reddish the further the light travels through the liquid. You may have to shine the torch into the side of the bottle to make a clean enough beam to see this.

Sunset in a glass


Scattering in a bottleThe milk consists of tiny droplets of fat in the water. The white light coming from your torch  is made up of all the colours of the rainbow. When the light hits the tiny fat droplets some of it scatters (bounces randomly) off them.

The bluey colours are scattered more quickly than the redey colours (with green somewhere in between) this means that if you shine the light through a lot of mixture all that gets through is the red light.

Right at the bottom there is lots of blue light scattering out of the sides so it has a blue tinge, as you go through more milk the blue light runs out so there is just red and green scattering so it looks yellow and then orange.

How does this relate to the real world?

The air molecules and any dust in the atmosphere act in the same way as the fat globules in the milk scattering more blue light than red. The sun actually looks white from space but if the light goes though some atmosphere some of the blue scatters out leaving yellow, which is why the sun looks yellow.

At sunset or sunrise or sunset the light grazes though the atmosphere at a low angle and so travels through much more atmosphere, so the only light that gets though is red, hence sunsets are red.

If there is lots of dust in the atmosphere, because of fires or volcanic erruptions then there are more particles in the atmosphere to scatter the light so the sunset is even more intense.

A sunset

If you look up in any direction other than up at the sun the only light you can see is the light that has been scattered, this is mostly blue, so the skit is blue.

Blue Sky

Why does blue scatter more than other colours?

Light is a wave, it is part of the electromagnetic spectrum which includes radio waves, microwaves, X-rays, gamma rays etc. Like any other wave, light waves have a wavelength ( 400 - 700nm ) light with different wavelengths has different colours. The long wavelength is equivalent to red and the short, blue (with green in between).

Red light's wavelength is significantly longer than the size of the particles. If you imagine a post in a harbour and a long wavelength ocean swell coming in, the post will just be too small to make much of a reflection (the difference in wave height between the two sides of the post is tiny so there is nothing to reflect).

Long wavelength Scattering

however if there is a much shorter wave coming in (bluer) that is more similar to the size of the post, you will get much more of a reflection.

Shorter wavelengths are scattered more

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