Nugget of Knowledge: Fluorescence & Phosphorescence

I was taught that we can see objects as light falling on them is reflected and enters our eyes. Now, I was very freaked out at this concept of things entering my eyes without me knowing about them. So I shut my eyes tightly to prevent the light from entering them.

I was also taught that luminous objects are the ones which have their own light. Thinking to myself I found out light cannot be created out of nothing. We need energy ,like electricity in case of a light bulb or chemical energy in case of a fire, or matter, like in nuclear reactions or in the sun, to create light. So I wondered how can light be of their own.

But one day to my amusement (and horror) my doctor gave me a wrist-band for not crying when being vaccinated. A wrist-band of this kind-


This thing had the ability to glow all night without any electricity, gas or fuel! Strange.

I also had this pre-assumption that things which glowed became hot. Like the electric bulbs in my house heated up, or the flame of cooking gas was hot and then there was the very-very hot sun! To my amazement this wrist band never turned hot, or even warm. But still it glowed all night; day after day. Weird.

Finally, I guess I have found out the reason for this – phosphorescence. 🙂 But before we go to it let us discuss fluorescence and luminescence.

Luminescent objects give out light which does not result from heat. It is a type of cold body radiation. It is mostly caused by chemical reactions and sub atomic motions.

On the other hand, the light resulting from heating of substances is called Incandescence.


Fluorescence is a special type of luminescence. The key difference is that the glow in this case is because of a particular type of sub-atomic motion ( by electrons). Electrons take in energy (normal light) and then somehow modify its wavelength and return it to us. This changes the colour and tone of the normal light to a unique glow.


But the problem in fluorescence is that it is short-lived. The receiving and emitting of light (radiation) is almost simultaneous. And hence fluorescent materials lose their glow after a few seconds. But the wrist-band that it started it all for me could glow all night! This is where phosphorescence comes in.

Phosphorescent materials absorb the radiation for a long duration and release it gradually. Hence they keep glowing for a very long time when compared to fluorescent materials. To explain this to you in a simple language I’ll strike an analogy. Think of electrons in atoms of phosphorescent substances to gain energy and move up the energy ladder. But when the try to come down after you switch the light off they are not allowed to because of some restrictions that quantum physics places on them. Hence it takes them hours to go back to their original place again and so the glow also continues for hours.


Interesting right? 🙂 I bet you will never look at glowing things again with the same eye!