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Black or White?

For some science experiments, the explanation isn’t black and white, but in this case, it is!

Australian Curriculum links

  • Science > Physical Sciences > Year 3 > ACSSU049
  • Science > Earth and Space Sciences > Year 7 > ACSSU116

You’ll need

  • Two empty cans of soft drink: one white/silver can and one black can
    • Diet Coke and Coke Zero work well, or you can always paint your cans black and white
  • A heat source e.g. a heat lamp or the sun
  • Two thermometers

Try this

  1. Place a thermometer in each can and take an initial reading of the temperature. Both temperatures should be the same.
  2. Place the two cans (with thermometers) in the sun or in front of the heat lamp. Make sure they are the same distance from the heat source
  3. Monitor the changes in temperature over a few minutes. Does one can increase in temperature at a greater rate?

Further investigation

  • Try using different coloured cans. Which colours give the greatest variation in temperature change?
  • What happens if you fill the cans with water? How does this vary the change in temperature?
  • What happens if you perform the experiment with unopened cans of soft drink? Does it affect the temperature of the liquid inside?
  • If you refrigerate a black and white can that start at the same temperature, is the cooling rate affected by the can colour?

What's happening?

The sun (or heat lamp) emits light, which is a form of energy. When the light hits a dark object, like the black soft drink can, it will mostly be absorbed and this energy is converted into heat. When the light hits a light coloured object, like the white soft drink can, it is mostly reflected, which means only a small amount of the light energy is converted into heat.

In the real world

The reflectivity of objects can be described as their ‘albedo’. The earth has a global albedo of about 30%, which means it reflects about 30% of the sun’s radiation. Snow covered areas such as the North and South Poles reflect more of the sun’s UV radiation back into space. Clouds also are very good at reflecting the sun’s light before it reaches the lower atmosphere. Areas of dark dense forest tend to absorb a lot of the sun’s UV radiation, warming the surface.

Cloud activity, deforestation and melting of ice sheets can work on a large scale to change the Earth’s global albedo. Large volcanic eruptions release large sulphur clouds into the atmosphere which increase the albedo effect by reflecting the Sun’s radiation. Throughout history, declines in the Earth’s global average temperature have aligned with major volcanic eruptions.