What Exactly Is UV Or 'Black' Light? And Why We Humans Are All Colour-blind....
Updated: Jun 20, 2018
When it comes to the full electromagnetic radiation spectrum thrown out by the sun, us humans can only see a tiny portion known as 'visible' light. Curiously, it seems we are in minority when it comes to seeing the full picture - or should I say the full spectrum. I took a peek into the hidden world of UV light and found creatures whose eyesight has evolved to see well into the UV spectrum, and humans who have developed superpowers after eye operations. So what exactly are we missing out on?
When most people think about UV light, they are probably aware of its association with sunscreen, or maybe even the eerie purple lights in nightclubs that make your teeth glow. If you were thinking there is a connection between UV and sunlight, you'd be absolutely correct.
It all begins with the sun and the electromagnetic radiation it produces.
The sun provides us with the necessary light energy and heat to sustain life here on earth. About 3% of the sunlight that makes its way to the earth's surface on a sunny day is ultraviolet light. For us humans, a suntan (or sunburn) are most obvious effects of over-exposure to UV – hence the link with sun-cream. And what about the weird purple lights in clubs? The UV range sits just outside the range of 'visible' light - the rainbow of colours from red to purple humans can see, at the purple end of the spectrum.
Interesting factoids about UV light – also known as black light.
Here are 4 important facts that shed more light on this mysterious spectrum:
1. The UV light spectrum can be divided into three bands depending on their energy levels:
UV-A is 315-400 nm - the spectrum that many black light bulbs are produced in and composes most of the UV on the earth's surface.
UV-B is 280-315 nm and very dangerous for our skin and eyes, hence the need for sunscreen and sunglasses. Fortunately UV-B represents only 2% of the overall flux on Earth.
UV-C sits between 100-280 nm. It does not reach the Earth’s surface due to being absorbed by our atmosphere.
2. UV rays are actually almost entirely invisible to most humans.
This is also the reason it is known as 'black light'. The electromagnetic spectrum of UV light is between 10 and 400 nm and humans can only see a range of 390 – 700 nm. The reason most ultraviolet rays are invisible to most humans is that the lens of the human eye blocks much of the radiation in the wavelength range of 300–400 nm.
3. So how is it we can just about see it when we are in a nightclub or enjoying an episode of CSI, I hear you ask?
UV light actually just sits inside the spectrum of our vision by about 10nm and our photoreceptors in the retina are still sensitive to 'near-UV' (300-400 nm). Ultraviolet lamps emit a whole range of light but our eyes can only see a small part of the reflected light in the purple spectrum, and only when we exclude all other visible light. That's why when using a UV bulb all other light sources need to be switched off for maximum effect.
4. Near ultraviolet (300-400 nm) is actually visible to many insects, birds and fish.
A bee, for example can see between 300-650 nm. This is well beyond the 300-400 nm us humans can see.
Are we really colour-blind compared to other creatures?
In my post on colour-blindness I delved into the world of human vision and how we perceive colours. Special cells on our retinas called "photoreceptors" (containing rods and cones) are responsible for colour vision. Our cones are sensitive to long (red - R), medium (green - G), or short (blue - B) wavelengths of light.
The presence of three fully functioning types of photoreceptors gives us full colour vision.
Remove any one from this trio of cones and a person will suffer from colour-blindness.
In the same way that having a missing or defective photoreceptor changes the colours that colour-blind people see, having an extra photoreceptor changes the colours that birds for example, can see. Birds are amongst a large number of lucky little critters that have an extra type of photoreceptor particularly sensitive to UV light.
A few years ago a butterfly was discovered with a staggering 15 types of photoreceptor.
Colour and markings are commonly used as a signal in animal communication.
In the case of butterflies it's a defence mechanism, disguising themselves through mimicry to deter preditors such as birds. But it's a double-edged sword: butterflies find it problematic to find one another when it comes to mating.
Evolving an extra photoreceptor or two has allowed certain species of butterflies to survive by aiding their search for mates.
Arctic reindeer are also known to have evolved sight well into the UV spectrum enabling them to effectively forage for white tree moss on snowy winter nights.
By being able to see which surface reflects UV (appearing to be white) or absorb UV (showing up black) they can spot the moss that sustains them or the wolves that prey on them – both of which stand out as black in an otherwise white environment.
The good news for us is that we won't have to go through millions of years of evolution to get a view into the realm of wildly fluorescing colours. A number of talented photographers equipped with special high tech UV cameras such as Craig Burrows and Dr Claus Schmitt have provided us a portal into the stunning world of insect vision.
Just typing 'flowers under UV light' into a Google Images search will make your mind boggle.
How some people can see into the UV spectrum.
There are stories abound on the internet how some people see the world with a UV blue/violet glow that adds an extra fun dimension to their lives. These people are not however, superhuman. They were not born with it either.
The clue to how they developed their supervision is in the lenses of their eyes, or more specifically the cornea.
Our ability to see in the UV spectrum is thwarted by the fact that our eyes block most of the radiation in the wavelength range of 300–400 nm range (near UV).
However any eye operation that requires the natural cornea to be replaced, such as cataracts, with artificial alternatives removes this energy filtering barrier. This leaves these people with a vision spectrum more akin to night-hunting raptors, some types of lizard and even ants. It is cool, but it's not a miracle.
The entire subject area of ultraviolet light and our vision is immense and interesting. This post is but a mere peek into it. In the coming months I hope to write more on the topic of animal vision and UV. In the meantime if you want to know more about the hidden world of UV light, why not check out this article of the list of things that glow under UV light or buy a UV bulb and see for yourself the world in UV.
Do you have any interesting facts about UV you would like to share? Let us know in the comments sections below - we'd love to hear them.