Heat energy moves from hot objects to cold ones by conduction, convection and radiation. Of these three, only radiation does not require contact; the sun warms the Earth because its heat radiation travels through empty space. Any warm object, such as the sun, a toaster or the human body, gives off this energy, called infrared radiation, or I.R. Simple experiments show you how it works.
A radiometer, available at science shops for a few dollars, shows the energy inherent in infrared radiation. It has a sealed glass envelope, like a clear light bulb, containing a partial vacuum. Inside the envelope, four square vanes, with a black side and a white side, balance on a needle bearing. When you shine a light on the radiometer, the black side warms more than the white side. Air molecules bounce off both sides, but the heat energy from the black side makes them push harder. This spins the vanes in the direction of the white side. Place the radiometer in bright sunlight and see how fast it spins. Then move it away from direct sunlight and observe that it moves more slowly.
Coloured Cloth in Sunlight
Find several cloth items, such as shirts or towels, of various colours. Lay them on a sturdy, level surface in the bright sunlight. After 15 to 20 minutes, feel each one and note which is the warmest. Because dark colours reflect the least sunlight, they absorb the most heat. Light colours reflect the most sunlight and therefore stay cooler.
Coloured Cups in Darkness
Gather five coffee cups, identical except for colour. Run hot water from a tap for a minute or two, until it reaches its maximum temperature. Fill the cups with hot water and move them to a dark, cool room. Place a thermometer in each one and wait 20 minutes. Read the thermometer in each cup and compare the temperatures and colours. The darkest colours should read the coolest because, just as they are better at absorbing heat energy, they also radiate heat energy more efficiently than light colours.
Obtain a "solar tube" balloon and take it outdoors on a calm, sunny day. The balloon is several feet long and made of dark plastic. Inflate it in the shade with cool air. Affix a kite string to the balloon and take it to a sunny area. Hold onto the string. Eventually, heat radiation from the sun will cause the air inside to expand, making the balloon rise from the ground. The warmer air inside has less density than the outside air, so the balloon floats.