Light is part of a spectrum called the Electromagnetic Spectrum, which also includes Gamma Rays, X-rays, ultraviolet and infra-red radiation, microwaves and radio waves. 

The Electromagnetic Spectrum is the way scientists refer to a stream of energy (photons). Photons move in waves. The gap between these waves is governed by how much energy the photon has. Big gaps (long waves) indicate lower energy and small gaps (short waves) indicate higher energy. To make it easier to understand, this stream of energy is split into groups according to the gap between the waves - the "wavelength".
Radio waves (long wavelength, low energy) can have as much as a kilometre between each wave whereas at the other end of the spectrum, with visible and ultraviolet light (short wavelength, high energy) the gap is so small it's measured in nm (nanometers – 1 thousand of a millionth of a metre!).

The human eye can see radiation with wavelengths from 400 to 700 nanometers (nm), and so we refer to this as "visible light". Ultraviolet light has a shorter wavelength than visible light, and cannot be seen by humans although for many animals, including reptiles, vision extends well into the ultraviolet.

On the diagram below you can see how ultraviolet light fits into the electromagnetic spectrum.

Traditionally, ultraviolet light is divided into three categories, UVA, UVB and UVC.

UVA is part of the visible spectrum for reptiles; they see colours and patterns differently to us because of this extra dimension to their vision. Some reptiles rely upon UVA light to identify individuals of their own species by their UVA-reflective markings; many plants and insects also have distinctive UVA reflectance and "patterns" which enable reptiles to recognise them.25,28,31,40,41

Reptiles exposed to UVA light show increased social behaviour and activity levels, are more inclined to bask and feed and are also more likely to reproduce as UVA light has a positive effect on the pineal gland, a light-sensitive structure just below the brain which responds to the increase and decrease of daylight with the changing seasons.2,36

UVB (280-320nm*) is found in natural sunlight. The atmosphere blocks wavelengths below 290nm so on the earth's surface, the UVB range is from 290 - 320nm. UVB is blocked almost completely by ordinary glass and by most plastics, so it does not pass through windows or the sides of glass vivaria.

It is not provided by normal household lighting or most so-called "full spectrum" lights, but nowadays there is an ever improving and expanding range of lights that can supply UVB in the vivarium.

There is growing evidence that reptiles can actually detect UVB, although whether it is actually visible to them is uncertain.15

Many species of reptiles, in particular diurnal lizards which bask in sunlight, utilise UVB radiation, in the region of 290 to 315 nm, to facilitate the photo-biosynthesis of pre-vitamin D3 (cholecalciferol) in the skin. If such reptiles are deprived of this particular wavelength of ultraviolet radiation, they are at risk of developing vitamin D deficiency, which may be manifested as metabolic bone disorder, a crippling and often fatal disease seen all too often in larger lizards such as iguanas and bearded dragons.1,6,7,8,27,35

UVB may have other beneficial effects. It has been shown to stimulate the production of beta-endorphins in human skin, resulting in a sense of well-being.22 There is no reason to suppose this process occurs solely in humans.

For more information on natural ultraviolet light and to read about our studies, continue to:
Using the UVB meter
UV light in nature
What UV light do reptiles need?

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