What is fluorescence?
The cup in the picture below contains a liquid that looks colorless under normal light. On the right, the cup is illuminated witha blacklight. Blacklight emit ultraviolet light and appear dark violet. Most of the light coming from the blacklight is not visible to our eyes and the camera. But the liquid in the cup can absorb thus ultraviolet light and emit blue light instead. This conversion of light from one color to another is done by fluorescent molecules. The cup contains tonic water which contains the fluorescent molecule quinine.
In our everyday lives we encounter many different fluorescent compounds. Like quinine, they absorb light of one color (often blue or ultraviolet) and emit light at a different color. Fluorescence is used in paper and laundry detergent to make colors appear brighter. It is also used as a display by animals and plants to make them look more interesting and attract attention. Of course, humans can do that too, as you can see in the costumes in the picture below. Fluorescence is also important in science, for example to make parts of a cell visible under a microscope, and in medicine to show surgeons where tumors, blood vessels, or nerves are located during a surgery.
Light is made up of photons. The color of light is related to the wavelength or energy of each photon. Fluorescent molecules can absorb a photon of one energy. This causes an electron in the molecule to go into an exited state. In a normal material, the electron would after a while just return to the ground state and return the absorbed energy by emitting a photon of the came color. Fluorescent molecules can instead turn some of the energy into heat or othe forms of energy and move to a different lower excited energy state. From there they can then return to the ground tate and emit a photon of lower energy (and therefor a different color) than the photon that started the process. The whole procedure is shown in the cartoon in the Figure.
The emitted photon contains less energy than the absorbed one, so the emitted light has a longer wavelength. In ome molecules the emission happens very fast. This emitted light is fluorescence. Some molecules can remain in the excited state for hours or even longer and glow long after they have been exposed to light. This is called phosphorescence.
UV light is invisible to the human eye, but the fluorescence falls into the visible light range as it has longer wavelength. It is impossible to see the blacklight, but the blue fluorescence from tonic water is noticeable.
The absorption and emission happen in certain wavelength or energy ranges that are different for every molecule. In this figure, the red line shows the amount of light the quinine in tonic water can absorb as a function of to the light wavelength. Similarly, the blue line tells how much light it can emit compared as a function of wavelength wavelength. The absorption peak is outside the visible light range, but the emission peak falls on the blue color. That is why the tonic water looks blue under UV light.
Bioluminescence is a type of luminance that causes the emission of light through a living organism. We can see this happen with fireflies, railroad worms, jellyfish, angler fish, and specific fungi (mushrooms). The living organisms produce energy by chemical reactions and emit light, thus, an excitation light sourceis not required. The bioluminescence also provides a protection against predators for these specific organisms.
Phosphorescence is closely related to fluorescence, but the main difference is the spin state of the excited electron reverts, which prevents it from dropping back easily. As a result, this process lasts much longer (even a few hours) than fluorescence. This is seen in glow sticks, as well as in glow-in-the-dark objects or toys!
