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About "Ask A Scientist!"

On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 14 August, 2008 Next Week's Question
Electron excitation responsible for colors of objects
Question
What is color? I know that objects absorb some colors and reflect others. But why do objects absorb or reflect certain colors? What do you add to something to make it reflect a color?

Question
In order to understand the colors of objects, we must first appreciate something about the nature of visible light. Since light acts as a wave, it may be characterized by its wavelength, ranging from about 400 nanometers for violet light (where one nanometer equals one billionth of a meter), to about 700 nanometers for red light. The other colors of visible light, and their wavelengths in nanometers, are blue (470), green (530), yellow (580), and orange (620). The energy of light depends upon its wavelength; shorter wavelengths have higher energies than longer wavelengths.

The light produced by most lamps, and by the sun, contains all visible colors, and is perceived as white. When an object is illuminated by white light, it may be reflected, transmitted, or absorbed. It is the wavelengths of the reflected and transmitted light that determine the color that we perceive. For example, the leaves of most plants absorb red and blue light very strongly and reflect green light efficiently. Carrots, on the other hand, absorb blue and green light and reflect yellow, orange and red light. So we perceive leaves to be green and carrots as orange. The exact shades of the colors that we observe depend upon the precise wavelengths that are absorbed. Smooth and rough surfaces reflect light differently, making some objects appear shiny and others dull.

Most objects are made up of molecules, which in turn consist of individual atoms held together by electrons. It is the electrons in atoms and molecules that are responsible for the absorption of light. For example, when the molecule chlorophyll in plants absorbs blue or red light, an electron is excited from a low energy state to a higher energy state. The energy difference between the two states must be exactly equal to the energy of the absorbed light. In the case of chlorophyll, the energy gaps between the lower and higher energy states correspond to light that appears either red or blue to our eye, and the unabsorbed green light is reflected. On the other hand, the energy of light absorbed by electrons in carotene, the molecule responsible for the color of carrots, corresponds to wavelengths perceived as blue and green, and orange and red light is reflected. The carbon in charcoal or in a pencil absorbs all visible colors. Very little visible light is reflected, so these materials appear black. White paper and white cloth do not absorb visible wavelengths, but instead reflect all of them.

Highly colored molecules that absorb at certain wavelengths, known as dyes and pigments, have been used for thousands of years to add color to materials. While chlorophyll is a common pigment easily extracted from plants, it isn't commonly used for coloring clothes because it is somewhat soluble in soapy water. That fact will be useful to people who get grass stains on their clothes!