Figure 15.9: Waves. The high points are the crests, the low point is the trough. The
wavelength is the distance from crest to crest. ( 27 )
sources are the Sun and a light bulb. Some objects radiate electromagnetic waves at wave-
lengths that we cannot see. The glass of water sitting next to you does not radiate visible
light, but it does radiate a tiny amount of heat.
You should be aware that some objects appear to radiate visible light, but they actually do
not. The moon and the planets, for example, do not emit light of their own. They reflect
the light of the Sun. Reflectionis when light bounces back from a surface. Albedois a
measure of how well a surface reflects light. A surface that reflects a high percentage of the
light that strikes it has high albedo and one that reflects a small percentage has low albedo.
One important fact to remember is that energy cannot be created or destroyed. It can only
be changed from one form to another. In photosynthesis, for example, plants convert the
Sun’s energy into food energy. They do not create new energy. When energy is transformed,
often some becomes heat. Heat transfers between materials easily, from warmer objects
to cooler ones. If no more heat is added, eventually all of a material will reach the same
temperature.
Temperature is a measure of how fast the atoms in a material are vibrating. High tempera-
ture particles vibrate faster than low temperature particles. Rapidly vibrating atoms smash
together, which generates heat. As a material cools down, the atoms vibrate more slowly
and collide less frequently. As a result, they emit less heat.
What is the difference between heat and temperature? Temperature measures how fast a
material’s atoms are vibrating. Heat measures the material’s total energy. Think of a candle
flame and a bathtub full of hot water. Which has a higher heat and which has a higher
temperature? Surprisingly, the flame has a higher temperature, but much less heat because
the hot region is very small. The bathtub has lower temperature but contains much more