January 10 1

400 500 600 800

January 10 2

Artist color wheel showing the colors which are complementary to one another and the wavelength range of each color.

January 10 3

January 10 4

Black & White

January 10 5

750!

430!

650! 580!

560!

490!

400!

January 10 6

Solutions of common transition metal compounds showing various colors.

January 10 7

Recording the absorption Spectrum

January 10 8

We have seen that light is a form of energy carried by electric and magnetic fields traveling at 186,000 miles per second. Light has both particle and wave characteristics. The wavelength of the light deter-mines both its color and its energy; the shorter the wavelength, the higher the energy. Light, also called electromagnetic radiation, ranges in wavelength from gamma rays (10 15 m) through the visible region (500 nm) to the radio wave region (100 m). In the visible region, white light contains a spectrum of wavelengths from 400 nm (violet) to 780 nm (red); these can be seen in a rainbow or when light passes through a prism. The color of substances depends on the color of light absorbed by the molecules or atoms that compose the substance. This, in turn, depends on the energy separations between electron orbits. When a molecule or atom absorbs light, electrons are excited from lower energy orbits to higher energy orbits. If the energy of the light is high enough, light can break chemical bonds and destroy or change molecules through photode composition; usually, however, the energy is simply given off again as heat or light through relaxation. The specific wavelengths at which molecules or atoms absorb or emit light serve as fingerprints for specific substances, making spectroscopy—the interaction of light and matter—a useful tool in identifying unknown substances. Magnetic resonance imaging and laser devices are two important applications of light and its interaction with matter. Light is a fundamental part of our lives; by it, we see everything that we see. Sunlight is what keeps the earth alive and is our ultimate energy source. Human eyes can see a narrow band of light called visible light, but humans use many other wave-lengths for various purposes: X-rays and gamma rays are used in medicine, infrared light is used in night vision technology, microwaves are used in cooking, and radio waves are used in communication. The human eye evolved to see not only different intensities of light (black and white vision) but different colors as well. The colors that we see depend on the interaction of light with the molecules or atoms within the thing we are looking at. Just as we can identify some things by their color in the visible region of the spectrum, so scientists can identify substances by their “color” in other parts of the spectrum; this is called spectroscopy. Spectroscopy is used to make measurements important to society. For example, ozone levels in the upper atmosphere are monitored by spectroscopy, and magnetic resonance imaging (MRI) is a form of spectroscopy by which doctors image internal organs. The development of technology such as MRI is extremely beneficial to humans, but it also raises difficult questions—who should benefit from that technology? Does the high cost of technology leave some people unable to afford it? Is that fair? The ability to make concentrated and pure forms of light with lasers has also impacted society. From CD players to supermarket scanners to laser guided bombs, lasers have changed the way we live in the 40 years that have passed since their discovery.