A lit incandescent bulb has one predominant feature. Sure, it’s light in terms of its weight. And it’s fragile. It’s even bright. But, more than anything else, it’s hot.
Heat is a measure of how energetically atoms are moving within an object. Even in solids, atoms move around, bumping against each other. If the atoms within an object are relatively stationary, the object is cool, while a hot object is full of atoms sliding back and forth. In a burner on an electric range, for example, electrons push through the coil, crashing into other atoms along the way. These collisions put the electrons in excited states, and they release that energy as they transition back to a lower energy state.
A River of Electrons
The electrical current is kind of like a river, with some electrons flowing quickly and smoothly, while others bump and crash their way along. Then electrons that have been crashing along find some open space and rush by, while some of the quickly flowing electrons crash and slow down. So at any instant some electrons are moving very quickly, while others are moving slowly—but most are toodling along somewhere in between.
That speed profile corresponds to energy, so some electrons are at relatively low energy, a lot at slightly higher energy, and far fewer at the highest energies of all. The electrons can’t give up more energy than they have, so the emitted photons follow that same energy profile. Photons with low energy are at infrared wavelengths — wavelengths that we feel as heat, rather than detecting through our eyes. But the higher energy electrons emit higher energy photons: visible light.
Lighting on a Curve
The wavelength spread of the light emitted by a heated object more or less follows what’s called a blackbody curve. The blackbody curve is what we expect from a hot object. We’re familiar with the glow of red-hot coals, we know the glare of the yellowish noontime sun, and we’re even familiar with the piercing white of molten metal. All those light sources emit almost all their light following the blackbody curve. The spread of wavelengths from far infrared to visible is determined by the temperature. The higher the temperature, the more visible light is put out. Also, as the temperature changes, the color mix of the visible photons changes also. There’s a certain temperature range over which the light is considered “white,” from the yellowish-white of about 2500 Kelvin to the bluish-white of about 10000 Kelvin. (Kelvins are a measurement of temperature usually abbreviated as "K." Room temperature is about 295 K, while a 350°F oven would be about 450K.)
That’s how an incandescent bulb works. Current runs through a tiny tungsten wire, which heats up to about 4600 °F (about 2800 K). To keep the filament from burning, it’s put inside a bulb filled with a neutral gas. The radiation follows the blackbody curve, meaning most of the energy goes into the infrared wavelengths — heat — rather than visible light.
Since incandescent bulbs follow the blackbody curve, the color of their light is determined almost entirely by their temperature. The 2800 K light is the warm yellowish-white most Americans associate with home lighting, similar in color to early morning sunlight. Halogen lights, for example, are specialized incandescents that run at a slightly higher temperature than standard incandescents, so their color is a little “whiter” than that of a standard bulb.
Just like efficiency, color is just something that “comes with” incandescent bulbs. Since those characteristics are not things that could be easily changed, it’s not something that has received a lot of thought -- that is, until compact fluorescents arrived on the scene.
To learn about the physics of fluorescent lights, continue to Zapping Atoms — How Fluorescent Lamps Work.
Readers may also be interested in reading, Not Your Grandpa's Bulb: The Physics of Lighting, or A New State of Lighting — Solid State Lighting from Light Emitting Diodes.
Sources
- Fowler, M. (2008). Black Body Radiation. In Modern Physics. Retrieved from Galileo.phys.Virginia.edu.
- Anon. (N.D.). The Nature of Light and Color. Retrieved from motion.kodak.com
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