http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Our Sun is a medium sized star located ninety-three million miles from Earth. And yet, scientists know with a great deal of certainty that at least two-thirds of the ninety naturally occurring elements on Earth are present in the Sun. The indispensable tool used by scientists to obtain that knowledge is spectroscopy.
Credit for our current understanding of spectroscopy goes back three hundred years. From the time of Isaac Newton (1642-1727) scientists have known that light from the Sun could be broken into the colors of the rainbow by a glass prism. Realizing that a spectrum could be produced was just the first step. Developing the ability to understand what caused the colors and realize what could be learned from them took hundreds of years.
In the early 1800s William Wollaston made observations of the solar spectrum which contained a few dark lines. His results were verified and improved soon afterward by Joseph von Fraunhaufer who recognized around six hundred such dark lines. Eventually, Gustav Kirchhoff studied the phenomena and realized that:
- luminous (light emitting) solids and liquids emit continuous spectra with no dark lines (all visible wavelengths are present)
- gases which are rarefied (placed in a container under low pressure with low density) and made luminous produce spectra containing bright lines that are called emission spectra (only selected wavelengths are visible)
- when light from a luminous source passes through a gas, some of the continuous spectrum may be subtracted out by the gas, creating absorption, or dark line, spectra (that is, some wavelengths are missing from the continuous spectrum).
http://www.physics.sfasu.edu/astro/color.html
Kirchhoff’s success at describing the processes involved in producing these three types of spectra provided scientists with a powerful tool. Scientists eventually learned the reasons why Kirchhoff’s processes worked. They learned that all wavelengths of light slow down as they enter glass from air. They further learned that each wavelength slows down a different amount is it enters. This slowing down causes the light wave to bend (refract) to give us the colors which Newton had observed. As time went on they realized that each element produced its own unique spectrum. This is often referred to as the “fingerprint” for that element.
Explaining why each element had a unique spectrum was no easy task. This was explained in the early twentieth century by Niels Bohr. He theorized that the spectrum of each element came from energy emitted by or absorbed by an atom’s electrons as they moved about the nucleus. Although Bohr’s explanation was fundamentally correct, it took decades of work by scores of other scientists to develop today’s refined explanation using quantum mechanics (statistical theory which explains the microscopic world of atoms and light).
As it turns out, it is electron transitions (movements between higher and lower energy levels) that produces both bright line (emission) spectra and dark line (absorption) spectra. The bright line (emission) spectra are produced when an excited electron (one which has absorbed energy) tumbles back down to its original stable location (its ground state) with respect to the nucleus. The dark line (absorption) spectra are produced when light from a white light source (all colors and wavelengths are present) travels through a gas. As the light travels through the gas, the electrons can absorb the wavelengths corresponding to the same wavelength that they produce when they are excited. For this reason the bright line spectra created by a hot gas will match exactly with dark lines seen in a spectra that was created when white light is sent through a cool sample of the same gas.
Astronomers can therefore use a star’s spectrum to determine which elements are present in its hot interior as well as which elements are present in its cooler exterior.
http://www.physics.sfasu.edu/astro/color.html
To continue learning about spectroscopy you can access the following links.
The following site provides additional information and images to help learn about spectroscopy.
http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
To access a site which allows you to view spectra for many of the elements go to:
http://www.colorado.edu/physics/PhysicsInitiative/Physics2000/quantumzone/lines.html
CREDITS:
Daniel Hortert GESSEP Program
Bennett Seidenstein GESSEP Program
Dr. Eric R. Christian ACE Deputy Project
Scientist
Beth Jacob ACE Outreach Specialist |
