ABOUT NIGHT VISION: There is very little visible light available to reflect off of objects at night, impeding human vision. A chemical called rhodopsin, found in the rod cells of the eye, is the key to good night vision. Whenever a rhodopsin molecule absorbs a photon, it splits into two secondary molecules -- retinal and opsin -- that later recombine back into rhodopsin. That's why most of us need a few minutes for our eyes to adjust to the dark when the lights go out. Exposure to bright light causes the rhodopsin to break down, impeding vision, until the two molecules combine back into rhodopsin. Unlike humans, cats have a special layer of cells lining the back of the feline retina, called the tapetum lucidum. It acts like a mirror, collecting and reflecting light back through the rods a second time to re-stimulate them. The result is a double exposure of light, permitting cats to see in near-darkness. When we see an eerie yellow glow in a cat's eyes in flash photographs, we are looking at light reflecting off the tapetum lucidum.
ABOUT LIGHT AND THE VISIBLE SPECTRUM: Light travels in waves, and what we see as different colors of light is due to the different wavelengths of light waves. Red light waves are longer than blue light waves. White light is made up of all the colors mixed together. Scientists use these properties in order to recognize or measure many things. Atoms and molecules in any given substance produce, absorb or change light in very unique ways; it is like a chemical fingerprint, telling scientists which atoms and molecules are present. In the same way, astronomers can tell what chemical elements are present in stars. Astronomers pass starlight through a special instrument called a spectrograph, which separates the light into the various colors of the spectrum, like a prism. By studying how the spectrum changes, scientists can tell which chemicals make up the star, and even its temperature, density and speed.