What is Luminescence?

Luminescence versus Incandescence

Incandescence is the emission of light caused by heat. When an object gains heat, the atoms gain energy and the electrons move from lower to higher energy states. The electrons release this energy as light (by emitting photons) so they can return to their original (ground) states.

Luminescence, however, is the emission of light by a source that has not been heated. The same principle of electrons changing energy states is at work in luminescent materials. I.e., luminescence works in the exact same way as incandescence, minus the heat. There are numerous types of luminescence, including triboluminescence, photoluminescence, chemiluminescence, and sonoluminescence.

Photoluminescence

Photoluminescence occurs when an object absorbs electromagnetic energy, or radiation, and releases it. There are two types: fluorescence and phosphorescence.

Fluorescence

This occurs when an object absorbs electromagnetic radiation and immediately emits it. Typically, the emitted light has less energy, but can emit the same or even higher energy than absorbed. If the energies are the same, it is called “resonance fluorescence.” 

Phosphorescence

Phosphorescence differs from fluorescence in that its emission is delayed. Not only do the electrons change energy state, but also their spin state, thus taking much longer to emit its absorbed radiation. The re-emission occurs for possibly hours and at a lower intensity. Glow-in-the-dark stickers, paint, etc., are phosphorescent.

Chemiluminescence

This occurs when chemical reactions excite electrons, which release light as they return to their ground states. One handy way to exploit this is finding blood in a crime scene. Some have used it for fun and made glow sticks.

In short, bending a glow stick breaks a small glass tube filled with hydrogen peroxide. This reacts with the diphenyl oxalate and dye mixture in the plastic tube. The reactions produces light! Since chemiluminescence is temperature dependent, refrigerating or freezing the glow sticks will make them last longer, but the light will be dimmer. This is because the lower temperature slows down the reaction. So as one would guess, higher temperatures would make them more vibrant and last for less time. This is not a suggestion to heat up glow sticks.

Bioluminescence

Humans aren’t the only ones who like shiny things. Bioluminescence is caused by luciferin in a living creature reacting with oxygen to produce light. Some reasons animals use bioluminescence include camouflage, attracting mates and prey, and defense. Many animals can deliberately change the colors and patterns of their lights. For example, the cuttlefish changes its colors to communicate and also to blend in with their environment. Other creatures, like anglerfish, use their natural light to lure prey.

Triboluminescence

Some materials will emit light when struck, broken, or rubbed. It is not yet fully understood, but it is theorized that the impact puts in enough energy to excite the electrons in the material so when they go back to their ground state, light is emitted. Others postulate that it is because the impact generates an electrical current in the atoms of the material. The current would flow through the material, exciting the gas within the crystal and exciting their electrons.

Geologist Dr. Hobart King mentions that triboluminescent materials can be used for detecting structural damage. “If triboluminescent materials are embedded in a composite, they will generate light if the composite begins to experience structural failure.” King continues, “A sensor will detect the light and report that failure has occurred. This monitoring can detect failure. . . far in advance of complete failure.”

Many may remember the chemistry lab with the Wint-O-Green Lifesavers. The teacher turned off the lights and the class would take pliers and crush the candy. Why? To see the spark it produced! When the pliers exert force on the candy, areas of positive and negative charge form in the crystal structures of the material. Electrons will jump across these gaps and react with the nitrogen in the air. Nitrogen cations are produced, producing blue light. (Fluorescence also occurs, due to the ultraviolet light produced reacting with the wintergreen oil molecules).

Sonoluminescence

When a field of high-pitched sound waves interact with a bubble, light is emitted. The theory that is most widely accepted is that the bubble glows due to shock waves concentrating energy in the bubble’s center as it shrinks.

Johns Hopkins University researcher Andrea Prosperetti offers an alternative theory. He says that a tiny jet of liquid shoots across the interior of the bubble at supersonic speed, slamming into the opposite side and fracturing the liquid. The energy is released as light. The jet of water is moving at over five times the speed of sound in air. Water molecules struck by this jet do not have enough time to flow away, so it fractures. It may be odd to think of a liquid acting as a solid, but note that this jet is moving near 4,000 miles per hour across a distance of 40 millionths of an inch. Prosperetti notes, concerning water, that “If you pull slowly, it just stretches or flows. But if you pull it really hard, it snaps, and you get a brittle fracture.” For more on this, visit this newswise article.

Main takeaway

For a quick reference:

  1. Luminescence: Light produced when electrons get energized (excited) and move into higher energy states. Light is emitted when they go back down to their original (ground) states.
  2. Incandescence: Same as luminescence, but electrons are energized by heat.
  3. Photoluminescence (PL): Objects absorbs radiation, then emits radiation (note: light=radiation).
  4. Fluorescence: PL with emission occurring immediately.
  5. Phosphorescence: PL but emission is delayed.
  6. Chemiluminescence (CL): Chemical reactions cause light emission.
  7. Bioluminescence: CL in living things.
  8. Triboluminescence: Objects light up when struck, broken, or rubbed.
  9. Sonoluminescence: Sounds causing light in bubbles either by a) shock waves concentrating energy in the center of a shrinking bubble, or by b) a jet stream moving from one side of the bubble’s interior to the other, fracturing the bubble wall.

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