Florescence: What is it?

Brightly coloured highlighters. Neon shirts. Things that seem to be glowing ever so slightly. These things are fluorescent, which means that the pigment that gives them their colour glows when light is shined onto them. Highlighters seem brighter than normal markers because they ARE brighter. Neon shirts look brighter because they ARE brighter. Lets take a look at how this works.

Fluorescent dyes are made of of special chemicals whose electrons can be excited by incoming light. When an electron of the dye is hit by and outside source of light, it may gain energy and go into an excited state. Here is stays for a split second, loses a tiny bit of energy due to vibration, then it falls back to the un-energized state, or ground state. But it still contains energy as it falls into the ground state, where does this energy go? This energy is transformed into a photon of light, and this escapes and is what we see when we observe fluorescence.

Most people only observe fluorescence with ultraviolet rays using a black light. But in reality, any colour of light can create fluorescence. So why do we choose ultraviolet light? The answer lies in the energy of the light. Fundamentally, electromagnetic radiation of higher frequencies such as ultraviolet light have more energy than lower frequency light such as red light. In order for something to exhibit fluorescence of a particular colour, it light of a higher frequency to push the electrons into that excited state. Green light has higher energy compared to red light but it has lower energy compared to blue light. This means that if we shine a green light on a red fluorescent dye, it will glow. But we will not get a glow if we shine the green light on a blue fluorescent dye. Why do we use ultraviolet light? Ultraviolet is higher energy compared to all visible light, so it is capable of causing all fluorescent dyes to glow. If we shines green light on a green fluorescent dye, then we will not get a glow. This is because once the electron gets excited it loses a tiny bit of energy due to vibration. This means that the light emitted will always be of lower frequency than the light absorbed.

Fluorescence vs. Phosphorescence (glow in the dark)
Fluorescence is very similar to phosphorescence, the process by which glow in the dark toys work. Both involve electrons being excited by light to higher energy levels, and both involve light being released when the fall back down. But fluorescence happens only when the electrons are constantly excited. If say the ultraviolet light is shut off, the glow goes away. But the electrons in phosphorescence do not all fall back at once so it may glow for minutes or hours after the initial light source is turned off.

This video explains this concept very well (along with some awesome visuals): 

Image Citation:
N.d. Principals of Chemistry. Web. 1 May 2016. <http://faculty.csupueblo.edu/linda.wilkes/111/3c.htm>.