Ultraviolet (UV) light or radiation is an electromagnetic radiation with a wavelength that is shorter than that of visible light but longer than that of X-rays. This spectrum consists of electromagnetic waves in the frequencies range higher than those identified as violet colors by human beings. Therefore, UV is not visible by human eye but has effects that people are aware of like the painful effect of sunburn among other beneficial and damaging applications. Infrared (IR) rays are a wide range of frequencies, starting those at the top of communication frequencies to those at the lowest frequency end (Red) of the visible spectrum. The fact that it comes just before the visible ray of red color in the visible spectrum makes it called Infrared.
UV rays are in the range of 10 nm to the 400 nm range and with energies ranging from 3 eV (electronic voltages) to 124 eV. For this reason, this type of UV light is classified as non-ionizing radiation (Rosinsky, 2006). Higher energies UVs, with 150 nm and above, are called ionizing radiations and consist of the vacuum’ UV but are not very penetrating and they are blocked by air. The wavelength is so because UV is longer than the X-rays but shorter than that of Visible light. It has velocity of (7.5 – 0.015) * 10^-14 Hz. IR have a wavelength longer than visible light (800 – 10^6) nm, a frequency of 3.0 – 0.0037 * 10^-11 Hz and include almost all the thermal radiations released by objects close to room temperature (Rosinsky, 2006).
Some of the applications of UV light are source of vitamin D, treatment of skin conditions like veiling, causes sunburns and indirect DNA damage. The applications of IR are treatment of chemotherapy, in meteorology for cloud height determination, research of global warming, and high heat may cause eye and vision problems.
Sunset comes around with the unique advantage of low-light to animals due to a structure of their eyes that allows them to function with optimum efficiency. In addition, nocturnal animals have more rods than cones in their eyes and this enables them to function well in dim light for the purposes of light assembling and perceiving motion. Again, light from far sources like the stars and the moon at night provide enough light to permit the animals to steer with much ease.
One amazing thing about animals in such conditions is that some animals like the rabbit exhibit red eye glow, the raccoon exhibit yellowish eye glow, while most members of the cat family glow greenish. This distinctive eye shine particular to various animals is due to tapetum lucidum meaning “Bright Tapestry”, the mirror-like layer of cells in or at the back of the eye meaning. In the presence of light, some light enters the eye, but some passes through and the tapetum lucidum bounces it back at the retina, offering the animal a second chance to see it (Rosinsky, 2006). It's this reflected light that gives off the glow we have come to associate with animals. This feature effectively increase the amount of light available for the animals eyes, and increase their ability to see in darkness.
A look at family photos may tempt one to think that the eyes of humans also glow due to the red-eye in them (Rosinsky, 2006). But no, this effect is usually associated with the camera flash bouncing off the red blood vessels and the red tissues behind the eye for all humans and animals without the tapetum lucidum. This is because the pupils have no time enough for it to contract against a bright flash of light.
Rosinsky N. M., (2006). Light: Shadows, Mirrors, and Rainbows. Picture Window Books. United States of America: USA.