How Does Fluorescence Occur? Detailed Explanation and Applications
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.
Conclusion
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 What Is Fluorescence in Physics?
1. What is fluorescence in Physics, and can you provide an example?
In Physics, fluorescence is a phenomenon where a substance absorbs light or other electromagnetic radiation of a specific wavelength and then emits light of a longer wavelength. The process is nearly instantaneous. It involves the absorption of a high-energy photon, which moves an electron to an excited state. The electron quickly returns to its ground state, releasing the absorbed energy as a lower-energy photon. A common example is a fluorescent highlighter pen, which absorbs invisible ultraviolet (UV) light from daylight and re-emits it as bright, visible colour.
2. What are some common real-world applications of fluorescence?
Fluorescence has numerous practical applications across various fields. Some of the most common ones include:
Lighting: Fluorescent lamps and compact fluorescent lamps (CFLs) use a fluorescent coating to convert UV light into visible light efficiently.
Biochemistry and Medicine: In fluorescence microscopy, fluorescent dyes are used to tag and visualise specific proteins or cells for diagnostic and research purposes.
Security: Many countries use anti-counterfeiting inks with fluorescent properties on banknotes, passports, and official documents, which are visible only under UV light.
Gemology: It is used to identify certain gemstones and minerals, as they exhibit characteristic fluorescence under UV light.
3. How does fluorescence differ from phosphorescence?
The primary difference between fluorescence and phosphorescence lies in the timescale of light emission after the excitation source is removed. Fluorescence is an almost instantaneous process where light emission stops as soon as the excitation source is off. This is due to a rapid electron transition from an excited singlet state. In contrast, phosphorescence involves a much slower, delayed emission of light, creating an 'afterglow' that can last for seconds or even hours. This delay occurs because electrons transition to a metastable triplet state, from which their return to the ground state is quantum-mechanically 'forbidden' and thus much slower.
4. Why does the glow from fluorescence stop almost immediately after the light source is removed?
The glow from fluorescence ceases almost instantly because the underlying atomic process is extremely fast. When a photon is absorbed, an electron jumps to a higher energy level, known as an excited singlet state. This state is highly unstable, and the electron returns to its stable ground state in about 10⁻⁸ seconds. This return journey, or de-excitation, releases a photon of light. Because this is a quantum-mechanically 'allowed' transition, it happens without any significant delay. As soon as the external light source is removed, no more electrons are excited, and the emission process stops immediately.
5. What is Stokes' shift in the context of fluorescence?
Stokes' shift is a fundamental principle of fluorescence that describes the difference in wavelength between the absorbed and emitted light. The light emitted by a fluorescent substance always has a longer wavelength (and thus lower energy) than the light it absorbed. This energy loss happens because the excited molecule loses some of its energy through non-radiative processes, such as molecular vibrations or heat, before it re-emits a photon. This energy difference ensures that the emitted photon is less energetic than the absorbed one, resulting in a shift towards the red end of the spectrum.
6. What conditions must a substance meet to exhibit fluorescence?
Not all substances can fluoresce. The ability to do so depends on a specific molecular structure and electron configuration. Key conditions include:
The substance must possess an electronic structure that allows an electron to be excited by absorbing a photon of a particular energy.
The molecular structure must be such that the de-excitation process favours the emission of a photon over non-radiative pathways like heat dissipation.
Often, molecules with rigid structures and conjugated pi-bond systems, like many aromatic organic compounds (e.g., quinine, fluorescein), are excellent fluorescent materials.
7. Can fluorescence be triggered by any type of light?
No, fluorescence cannot be triggered by any type of light. Each fluorescent substance has a unique absorption spectrum, which is the specific range of light wavelengths it can absorb to become excited. The incoming light must have sufficient energy (i.e., a short enough wavelength) to move the electrons to a higher energy state. For example, many fluorescent materials glow brightly under high-energy ultraviolet (UV) light but show no effect under lower-energy visible or infrared light because those photons lack the energy required for excitation.
