Ultraviolet light is invisible to humans although research has shown that bees use the UV emission from the sun to navigate between flowers and home. Some birds are equipped with a 4th optical sensor that allows complete detection of ultraviolet light. Though it’s adjacent to visible light on the electromagnetic spectrum, its wavelengths are just too short for human eyes to register and see. You would think that our retinas would activate on sensing UV but the UV light just can’t get through the lenses of our eyes. In fact, UV cannot even get through a pane of glass because the atoms in the glass composition absorb the UV energy like a sponge soaking up water.
Since UV waves have high-energy, we can feel their effects even though we cannot see the rays. For instance we know that ultraviolet light generated by the sun is what gives you a tan — or a sunburn if you stay out too long. In addition, if your body is exposed to UV for years by working outside all of the time without adequate protection, it may also lead to skin cancer.
Historically, UV light falls into two categories, long wave and short wave, covering the 400 to 100 nanometer wavelength range although new ISO standards offer up to six finer divisions. Let’s look at the long wave series first.
You can easily buy a “black light” that generates long wave ultraviolet. These were very popular in the 1970s when young people bought glowing posters complete with fluorescent paints that would glow bright orange or green. On a more useful note, if you are a rock-hound you know that some minerals can glow while being illuminated with long wave UV light (although a lot more will glow under short wave UV) and using UV sometimes helps in identifying similar looking rocks.