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The Challenge: Make an underwater flashlight

How can we get the atoms excited?

One way to get atoms in a metal excited is by putting an electric current through them. If you put an increasing current through a piece of metal it will eventually get red-hot. If you up the current a bit more, it will eventually get white hot. When a metal is glowing with heat, it is described as being incandescent.

Why is light given off?

In a red-hot object, the atoms are just getting enough energy to begin emitting the light that we can see. Red is the first visible light to be seen because it is the color which requires the least energy for emission. Once you apply a big enough current, you will energize all the electrons to such an extent that light of all colors is being generated. These colors mix together to appear white.