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Last updated date: 28th Feb 2024
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Introduction to Fluorescence

There are many fascinating phenomena that make us think twice and urge us to find the actual reason behind them. One such phenomenon is fluorescence. It is a remarkable phenomenon that occurs at the atomic level where energized atoms release electromagnetic radiation that comes within the visible range of humans. It can happen due to many reasons. In this section, we will learn what is fluorescence and how it happens.

What is Fluorescence?

It is an interesting phenomenon where a substance emits electromagnetic waves in the longer wavelength segment making them visible to naked eyes. To understand this, let us define fluorescence first.

When a fluorescent substance absorbs energy due to the incidence of X-ray, light waves, and electrons, it starts to emit visible light (longer wavelength) and slowly reaches the ground state from the partially excited state. This phenomenon is called fluorescence.

The substances that show such activities where they absorb energy from the incident electromagnetic waves falling in the ultraviolet range and emit light in the visible range of wavelength are called fluorescent substances.

Why Does Fluorescence Occur?

As per the fluorescence definition, when high-energy electromagnetic radiation falls on a fluorescent substance, it absorbs the energy immediately and starts showing this physical property. This happens when the atoms get energized and excited by the incidence of electrons, X-rays, or any other energy radiation. To reach the normal state, these atoms start losing energy in the form of electromagnetic radiation that falls under the visible range of human eyes. The emission begins within 10-8 seconds of the incidence of the high-energy electromagnetic rays or electrons.

The substance stops emitting light when the source of the electromagnetic radiation ceases. It can also absorb energy from UV rays and other electromagnetic radiation in this range. All the incident rays have a shorter wavelength and a higher energy level. The atoms of the fluorescent substance quickly lose their energy and stop emitting visible light when the electromagnetic source stops. Now that you know what is meant by fluorescence, let us learn a few examples first.

Examples of Fluorescence

As mentioned earlier, fluorescence is a temporary physical phenomenon where the atoms of a substance get energized and excited by the incidence of high-energy electromagnetic radiation. You can find them abiotic, as well as, biotic examples. For instance, minerals and gemstones often emit visible colors when UV rays fall on them. Diamond, rubies, emeralds, calcite, amber, etc. show the same phenomenon when UV rays or X-rays fall on them.

One of the best fluorescence examples in nature is bioluminescence. When the small bioluminescent phytoplankton is touched in the marine water, it starts to glow. Any kind of kinetic energy seems to charge the constituent atoms that make it glow. When there is no disturbance, you will not find any such physical activity.

Chlorophyll is also a fluorescent substance occurring naturally in green plants. This pigment is used to trap sun rays and convert them into energy by initiating a biochemical reaction. The reaction stops when the sun rays cease to exist. This pigment only shows such behavior in the presence of a particular range of wavelengths.

Difference Between Fluorescence and Phosphorescence

Another natural phenomenon that exhibits similar properties where a substance absorbs energy and the constituent atoms disperse it in the form of visible light is called phosphorescence. Here is a list of differences between phosphorescence and fluorescence.

  • In phosphorescence, the emission of visible light happens not instantly as in fluorescence. In fact, the emission of light continues for a longer time span. In fluorescence, the emission of light stops once the source of the higher energy electromagnetic waves ceases. It means when the source is cut down, fluorescence stops immediately whereas phosphorescence continues.

  • The lifespan of the excited atoms in fluorescence has a very short lifespan. They go back to their normal state very fast. In phosphorescence, the energized atoms take time to settle down to the low energy level.

  • The energy level of the emitted photon particles is lower than those were absorbed in fluorescence. The same happens in the phosphorescence but the wavelength of the emitted photons is longer than fluorescence.

  • The excitation in phosphorescence can be explained as an afterglow whereas fluorescence is similar to a flash that is momentary. The flash exists till the source emits electromagnetic radiation.

  • The perfect fluorescence examples are chlorophyll, jellyfish, vitamins, etc. Examples of phosphorescence are phosphorus, fireflies, clock dials, etc.


As per the fluorescence definition, you can now easily understand this phenomenon and how it happens. There are similar natural phenomena that can resemble this physical property. The differences between fluorescence and phosphorescence also make the concept clear. Study this physical property of certain fluorescent substances properly with examples so that you can understand the basic concept and utilize it to answer questions.

FAQs on Fluorescence

1. What is Fluorescence?

The natural phenomenon where a particular genre of substance emits visible light when electromagnetic radiation of a higher energy level falls on them is called fluorescence. It is used in different aspects. In fact, it has a separate branch in physics for study with the name fluorescent light physics. Researchers study the quality and features of the light emitted from these substances and utilize them to develop new conceptual things such as paints, accessories, etc.

2. What is Phosphorescence?

This is a physical phenomenon where the atoms get excited when exposed to high-energy radiation. The atoms take time to settle down and reach their low state level by emitting low-energy visible light. This is called phosphorescence. The best example is phosphorus. It continues to glow even when the source is cut.

3. Why Does Fluorescence Stop Immediately?

It takes only 10⁻⁸ seconds for a fluorescent substance to react and start radiating in the presence of high-energy electromagnetic radiation. It immediately stops glowing when the energy source ceases to exist. It is an instantaneous effect and a very special physical property of fluorescent substances such as chlorophyll, gemstones, vitamins, etc. 

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