Explore everyday household objects under ultraviolet (UV) light and discover the molecules that absorb invisible ultraviolet radiation and re-emit it as colourful visible light. Compare fluorescence with phosphorescence and find out how glowing materials are used in medicine, security and display technologies.
Have a go
In this activity, you can explore everyday household objects that glow under UV light. You can also look at some of the key molecules in them that absorb invisible UV light and re-emit it as colourful visible light. This process is known as fluorescence.
What's going on
UV light
Ultraviolet (UV) light is a type of electromagnetic radiation. It is found naturally in sunlight and has a shorter wavelength than visible light, making it invisible to the naked eye. We cannot see UV radiation, but our skin responds to it over time. There are 3 main types: UVA, UVB and UVC.
UVC is the shortest-wavelength and highest-energy form of UV radiation. Natural UVC rays from the Sun are completely absorbed by the ozone layer in the Earth’s atmosphere, preventing this radiation from reaching the surface.
UVB is high-energy UV radiation. Most is absorbed by the ozone layer. The part that reaches the Earth’s surface is the main cause of sunburn, because it damages the top layer of skin. Excessive exposure increases the risk of skin cancer.
UVA has the longest wavelength of the 3 types. It contributes to skin ageing, spots and wrinkles, and can penetrate glass. Our bodies reduce the amount of ultraviolet radiation reaching deeper tissue by producing more melanin.
Although our eyes cannot see UV light directly, some molecules can absorb its energy and convert it to visible colours.
How does fluorescence work?
When a molecule absorbs UV light, one of its electrons gains energy and moves to a higher-energy excited state. This is absorption. The excited state is only temporary. As the electron returns to a lower-energy state, some of the absorbed energy is released as visible light, producing the glow that we observe. Because some energy is lost during the process, the emitted light has a lower energy and longer wavelength than the UV light that was originally absorbed.
There are 2 different ways this can happen. In fluorescence, the excited electron returns to its original state very quickly, typically within a few billionths of a second. As a result, the material only glows while it is illuminated by UV light. When the UV lamp is switched off, the glow disappears almost immediately.
Sometimes the excited electron takes an alternative pathway into a longer-lived excited state before returning to the ground state through intersystem crossing (ISC). The electron becomes temporarily “trapped” and releases its energy much more slowly. Because the energy is released over a longer period, the material continues to glow after the UV light has been removed. This delayed emission is known as phosphorescence and is the principle behind glow-in-the-dark stars, toys, watch dials and some safety signs.
Many organic compounds contain conjugated systems, with alternating double and single bonds, which can allow electrons to move across the molecule. These delocalised electrons can absorb UV light more efficiently and emit visible light.
Everyday items that fluoresce
A selection of common items fluoresce under long-wave UV light. They look different from how we usually see them in normal light. They all contain molecules with electronic structures that can absorb UV radiation and re-emit visible light. Examples include turmeric, tonic water, antifreeze, olive oil, eggshells, honey and spinach.
| Item | What makes it glow? | Typical colour |
| Turmeric | Curcumin | Yellow |
| Tonic water | Quinine | Blue |
| Antifreeze | Fluorescein is often added | Green |
| Olive oil | Chlorophyll a and b and pheophytins | Red |
| Eggshells | Protoporphyrin IX | Red |
| Honey | A mixture of flavonoids, phenolic acids and aromatic compounds from nectar sources | Yellow, green, amber or orange |
| Spinach | Chlorophyll a, mainly | Red |
Why is fluorescence useful?
Small changes in molecular structure can change the way a molecule interacts with light, producing different fluorescent colours. By understanding these relationships, chemists can design new fluorescent materials for applications ranging from medicine to advanced technologies.
- Medical imaging: Fluorescent dyes help scientists and doctors observe cells, tissues and biological processes.
- Forensic science: UV light can reveal traces of substances that are difficult to see under normal lighting.
- Security features: Banknotes, passports and identification cards often contain fluorescent markings to help prevent counterfeiting.
- LED lighting and displays: Fluorescent materials help create bright colours in lamps, screens and display technology.
Fluorescence in transition metals and lanthanides
Organic compounds are not the only materials that luminesce. Transition metals and lanthanides can also luminesce, and they generally have longer lifetimes. In transition-metal compounds, the colour of the luminescence is highly dependent on the metal and the organic ligands that bind to it. Common examples include ruthenium (Ru²⁺) and iridium (Ir³⁺).
Unlike transition metals, lanthanides produce some of the brightest and most distinctive fluorescent colours. They produce sharp, line-like emission profiles with higher colour purity than the broad emissions of transition metals. This property makes them attractive for LED displays because they can provide higher colour contrast. Common examples include europium (Eu³⁺) and terbium (Tb³⁺), which emit bright red and bright green light respectively.
Metal-based fluorescent compounds are an important field of research because of their longer lifetimes and bright luminescence. Some can also be excited by visible light above 400nm, which can avoid autofluorescence from biological molecules and make them more useful for biomedical imaging.