Try this: Dark heat


	Secure the prism in place on the edge of a box or windowsill.


	Place black cardboard where the spectrum falls.


	Place a thermometer within the spectrum and one outside the red end.


	Watch the temperatures.

You will need

Three identical thermometers
Black marker pen
Black paper or cardboard
A triangular prism (from many hobby and games stores)
A box or window sill
Blu-tac or plasticine

What to do

Before you start, colour the bulbs on the thermometers with the black marker and leave them somewhere cool to dry.
Make sure the thermometers are all in a shaded spot together. Their starting temperature should be exactly the same.
Find a sunny space either outside in an area where there is no wind, or in front of an open window where the Sun can shine through.
Stick plasticine on one side of the prism and secure it to the windowsill. Alternatively, if you’re outside, place a box on its end and stick the prism on top, close to the edge.
Turn the prism so the Sun shines onto one of its flat sides. Now, tilt it on an angle so you can cast a rainbow, or ‘spectrum’, on the ground. The plasticine should help keep it in the right position.
Take the sheet of black paper or cardboard and place it over the spectrum, so the rainbow of colours shines directly on it.
Put a tiny bit of plasticine on the backs of each thermometer. Stick one of them directly into the spectrum so it lines up with either the blue or green colours. Make sure its bulb is well inside the light.
Place the second thermometer just below the red end of the spectrum, outside of the light, in the shade.
Place the third in shade a little way away from the spectrum.
Watch the temperature on each of them. Which increases the most?

What’s happening?

Oddly, the temperature on the two thermometers nearest the spectrum should increase, even though only one seems to be in the light. The one near the red end of the rainbow may even heat up slightly more!
While it might appear white, sunlight is actually a mix of all different colours – most of which are invisible to our eyes.
Light, or ‘electromagnetic radiation’, is a form of energy. It often acts like a stream of tiny particles. However it can also act like ripples in water and like any wave, it can wiggle quickly or wiggle slowly.
Light can be divided into groups according to its speed of wiggling, or ‘frequency’. At one end, the super-high frequency light waves consist of gamma waves and x-rays. At the other, the low frequency light waves consist of radio and microwaves. In between is a narrow range of frequencies our eyes detect as colour.
When a wave of light hits an atom, the atom will tend to soak it up. Depending on the light’s frequency, the atom will either keep the wave or spit it out again. By keeping the light wave, the atom turns it into another form of energy such as heat, which can be measured using a thermometer.
Light outside of the red part of the spectrum is called ‘infrared’, and has a lower frequency (and therefore less energy) than the other colours. However, the prism has an interesting effect on the light waves, concentrating the light at the red end of the spectrum and so heating your thermometer more.
We may not see anything, but there is light there, just as you can see by your thermometer’s temperature.

Applications

This experiment was first done in 1800 by the astronomer William Herschel. When measuring the temperature of various colours passing through a prism, he attempted to measure the room’s temperature by holding the thermometer outside of the red end of the spectrum. To his surprise, it was actually warmer (he didn’t consider the effect the prism had on concentrating the light in that area). He named this invisible light ‘Calorific Rays’, but it later became known as infrared.
There is a myth that says infrared light is a form of heat energy. In fact, all forms of light, when absorbed, will eventually become heat.
A large amount of sunlight that hits the Earth’s surface is reflected as infrared radiation. Some gases, such as carbon dioxide, water vapour and methane, absorb the infrared light and reflect it in a random direction. This sends a lot of it back to the Earth's surface, heating it. We call this process the Greenhouse Effect, as the gases act a little like the glass in a greenhouse.