Not Your Grandpa's Bulb: The Physics of Lighting

Mar 29, 2011
Richard Gaughan

Atoms are composed of a "cloud" of electrons surrounding a heavy central nucleus. - wikimedia commons/Furmanj

Today's consumer can choose from incandescent, fluorescent, and solid-state lighting. The science behind each drives their performance.

Currently, consumers have three options: incandescents, fluorescent bulbs, and the new kid on the block, solid-state lighting from light emitting diodes (LEDs). But incandescent bulbs are on their way out. The European Union and the United States have both enacted regulations on lighting energy usage that will eventually ban most traditional incandescent bulbs. Governments have nothing against old-fashioned light bulbs — except that a large percentage of the energy utilized by the bulbs use does not create light! The inefficiency of incandescent bulbs has very little to do with how well they’re made; it’s due to the fundamental physics involving light.

At the heart of each lighting method is the same basic process: an electron gets rid of energy, emitting a tiny amount of light. The color of the light depends upon the amount of energy the electron gives off; blue light has higher energy than red. Different light sources are distinguished by the different ways they put energy into electrons and the different way the energy comes out of the electrons.

The Basics

Here are the basics, things that are true for all normal light sources.

Just about all the light we’ll ever see comes from electronic transitions. Every atom is surrounded by a complement of electrons. Most of the time, the electrons are “comfortable” in their lowest energy states. An electron can absorb energy and move into a higher energy state, kind of like a baseball can accept energy from a player’s arm and fly into the sky. But just like that ball naturally tends to return to the ground, giving up its energy to the player’s glove, an “excited” electron returns to its lower energy state, giving its energy away.

One of the ways an excited electron can give away its energy is by emitting a photon, a tiny bit of light. How tiny? Well, a 60-watt incandescent bulb gives off about 4 billion billion photons every second (yes, that’s 4 billion billion). Each type of lighting works in this way: giving energy to electrons (“exciting” them) and having them release photons as they go back to their lower energy state. But incandescent, fluorescent, and LED lighting all excite electrons in different ways, and the photons they emit are correspondingly different. The efficiency (or inefficiency), the intensity, the color of a light source—all come back to the fundamental physics.

To learn about the physics of incandescent lighting, continue with Hot Bodies — How Incandescent Bulbs Work.

Or jump to the other articles in the Physics of Light series: Zapping Atoms — How Fluorescent Lamps Work or A New State of Lighting — Solid State Lighting from Light Emitting Diodes.