Waves, light and energy

We have seen that a wave is a means to transport energy from one place to another, in the space.
The properties of a wave (that is its frequency, its amplitude and wavelength) vary as the energy carried by the wave varies.
This energy depends on the energy entering the emission phenomenon: if an increase of energy is produced inside a body, it will try to release it, in order to reach a new equilibrium status. If a huge increase is produced, then it will emit radiation of great energy.
Otherwise, if the excess energy is small, then in turn the emitted radiation will carry little energy.

An increase of the energy carried by the wave can turn into an increase in the wave frequency (and thus into a decrease in its wavelength) or into an increase in its amplitude.

What does this "physically" mean? In order to understand what practically happens, think about a common phenomenon: the heating of a metal up to very high temperatures.
You did certainly observe, at least once, what happens to the electric resistance of an oven (which is a metallic wire) when it is switched on. Or else, you could see a casting of melted metal. At normal temperatures, the metal is opaque. When it is heated, after a while its color changes.
It starts to emit light, first red brown in color, then bright red. If the metal is heated until it melts (like in a blast furnace happens), it eventually gets a yellow and then a white color.

In the same way, you did observe that the wood embers in a fireplace, when they cool, turn from yellow to orange, then to red and to dark red, until finally they become gray when the cooling is complete.

What did change in the wood or the metal? The total energy they have! The embers loose their energy to the air and cool down, the metal gains energy when it heats up. Therefore the luminous waves that these two materials emit have properties that change in time as this energy changes.

Now think of the bulbs that you have at home: a 100 W bulb illuminates much more than a 25 watt bulb, but it also consumes much more electric energy. The energy implied in the light emission, is therefore different in the two cases. However, the light emitted by the two bulbs has the same color, contrary to the metal that we saw.

Think again for a moment at what we said about waves and light and answer a question.