Ultraviolet Light and Fluorescence:

If you look around you, you see objects of many different 
colours.  What your eyes are seeing are waves of light, 
but what is it that makes light from a red T-shirt 
different from light from a blue T-shirt?  Light is a type 
of wave, and we know that one distinguishing property of 
different types of waves is their wavelength (the 
distance between wave peaks).  This is what make red 
light different from blue � the red light has a longer 
wavelength.  

Human eyes are sensitive to only a very small range of 
possible wavelengths of light waves.  There is a whole 
spectrum of waves that are not visible to us, but that we 
make use of in many different ways.  For example, radio 
and television waves are used to transmit signals from 
the broadcast antenna of your local radio/TV station, 
which can be picked up by the aerial of a radio or TV set 
in your community.  Another example might be the heat 
you feel if you stand near a fire � you feel warm because 
energy is being carried to you by waves that are of a 
wavelength your eyes cannot see.

Light waves that have a wavelength slightly shorter than 
visible light are called "ultra-violet".  It turns out that 
they also carry slightly more energy than visible light 
waves.  

Physicists have studied what matter is made up of at its 
most fundamental level, and learned that atoms are one 
of the basic building blocks.  We now know that what 
distinguishes the different chemical elements is that 
they are made up of atoms with different numbers of 
electrons in their atoms.

By shining ultraviolet light onto certain elements, the 
energy of the light wave is absorbed by the atoms of 
that element, and some of that energy can subsequently 
be re-emitted, but this time with a wavelength that is 
visible to us.  This is called fluorescence.  Shining 
ultraviolet light onto white clothing (perhaps your shirt) 
is a good example of this.  Try it out, and see if you glow!