How Light Is Produced: Incandescence, Luminescence and LEDs

A firefly, a glow stick and a neon sign walk into a lab...

On a July night in Ontario you might see fireflies flashing over a field. At a Canada Day celebration, kids wave glow sticks. Downtown, a sign glows red-orange. All three make light, but in completely different ways.

An object that makes its own light is luminous. An object that we see only because it reflects light (the Moon, this page, your face) is non-luminous. In this lesson you will learn the main ways luminous objects produce light, and the vocabulary scientists use for each.

The big idea: excited electrons give off light

Almost every light source works the same way at the atomic level:

1. Energy is added to atoms. The energy might come from heat, electricity, a chemical reaction or light (often ultraviolet).
2. Electrons in the atoms jump to a higher energy level. We say they are excited.
3. The excited electrons fall back to a lower energy level. As they drop, they give off the extra energy as light.

The size of the drop decides the colour: a bigger energy drop gives light with a higher frequency (towards blue and violet); a smaller drop gives lower-frequency light (towards red).

1. Incandescence: light from heat

Incandescence is light given off by an object because it is very hot. Heat a stove element and it glows dull red; much hotter objects glow yellow-white. The Sun, candle flames, glowing campfire coals and old-style incandescent bulbs all produce light this way.

In an incandescent bulb, electric current heats a thin tungsten filament until it glows white-hot. The problem: most of the energy (around 90%) comes out as heat (infrared), not visible light. That is why incandescent bulbs are inefficient and why they have mostly been replaced.

2. Luminescence: light without high temperatures

Luminescence is light produced without the source needing to be very hot. There are several kinds:

• Electric discharge: an electric current passes through a gas, exciting its atoms. Each gas gives its own colour: neon glows red-orange; sodium vapour street lights glow yellow-orange. Lightning is a giant natural discharge. The aurora borealis (northern lights), often seen across northern Canada, forms in a similar way: fast charged particles from the Sun strike oxygen and nitrogen in the upper atmosphere and make them glow, often green.
• Fluorescence: a material absorbs ultraviolet light and immediately gives off visible light. It stops glowing as soon as the UV is removed. Highlighter ink and white T-shirts glow under a "black light". In a fluorescent tube or compact fluorescent lamp (CFL), an electric discharge through mercury vapour makes UV, and a phosphor coating on the inside of the glass turns the UV into visible light. CFLs contain a small amount of mercury, so they must be recycled, not thrown in the garbage.
• Phosphorescence: like fluorescence, but the material stores the energy and releases it slowly, so it keeps glowing after the light source is removed. Glow-in-the-dark stars and exit-sign paints work this way.
• Chemiluminescence: a chemical reaction produces light with very little heat. Bending a glow stick breaks an inner capsule, so two chemicals mix and react. Forensic scientists spray a chemical called luminol, which glows when it reacts with tiny traces of blood.
• Bioluminescence: chemiluminescence in living things. Fireflies, some fungi, many deep-sea fish and some ocean plankton make light using a reaction between a molecule called luciferin and an enzyme called luciferase.
• Triboluminescence: light produced when certain crystals are crushed, rubbed or broken apart. Crushing a wintergreen-flavoured hard candy in the dark, or quickly peeling some kinds of tape, produces faint flashes.
• Light-emitting diodes (LEDs): an electric current passes through a semiconductor. Electrons drop to lower energy levels and give off light of one colour, set by the material. Most "white" LEDs are actually blue LEDs coated with a yellow phosphor; blue plus yellow looks white.