Have you ever wondered why the sky is blue in the morning, and then orange in the evening? Incredibly, milk can show us why!

Large clear container lit from the side glows orange

You will need

  • Smooth sided plastic jar, like a tupperware container
  • Water
  • Milk (full cream works best)
  • Tablespoon
  • Torch, like the one on your phone
  • A darkened room

What to do

  1. Add 1 tablespoon of milk to your plastic jar.

  2. Fill your plastic jar up with water. Congratulations! You have made your own sky jar.

  3. Bring your sky jar into a room you can darken by closing the shades and turning off the light. The room should still be light enough for you to see outlines of objects in the room.

  4. Take your torch and place your jar in front of it. Be careful not to knock the jar over! What colours can you see in the jar? Do the colours change across the jar?

    Large clear container lit from the side glows orange
  5. Next place your torch on the table with the light shining up. Place the jar on top of it. What colours do you see?

    You can repeat this experiment and change the amount of milk in the sky jar. What changes when there is more or less milk in your sky jar?

    Large, clear container lit from underneath has a blue glow
  6. Once you’re finished, pour your sky jar down the drain. Please don’t drink it! It won’t taste nice.

What’s happening?

Sunlight – and the light from your torch – is a combination of all the colours of the rainbow. This light travels in waves, like the ones you might see at the beach. Some colours travel as short and choppy waves, like the colour blue. Other light waves are long and lazy, like the colour orange. When sunlight enters the Earth’s atmosphere, the light waves bump into gas particles that are in the air and scatter in different directions. Short light waves, like blue, are scattered the most.

In our sky jar, the water acts like Earth’s atmosphere, and the milk acts like the gas particles that scatter the light. When the sun is directly overhead, its light has a short distance to travel through the atmosphere. Over this short distance, the blue light still gets scattered the most. This scattering is why we see the sky as blue during the day. You can see this colour at the bottom of your sky jar, at the closest point to the torch.

Stunning Sunsets

As the Sun sets, the sunlight enters the atmosphere at an angle and must pass through more gas particles. This means that sunlight will collide with more gases and scatter more light waves. This completely scatters the blue light away, meanwhile the orange light makes its way through the atmosphere and into your eyes. This leads to the lovely orange colours we see during a sunset. You can see this at the top of your sky jar, the point furthest away from your torch.

You can experiment with your sky jar a lot. Test how moving the torch around and the distance it is from the jar affects the sky. See how adding more or less milk affects the sky in the same way more or less gases in the atmosphere would. With this experiment, the sky’s the limit.

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