Dawn's VIR spectrometer

What Can Scientists Learn From a Spectrum?

It took a long time for scientists to learn how to use a spectrum to gain insight into the universe. Isaac Newton (1642-1727) saw the spectrum from sunlight as a continuous band of colors. In 1802, William Wollaston (1766-1828) observed several dark lines in the spectrum of the Sun. He thought they represented natural divisions between the colors. Joseph von Fraunhofer (1787-1826) looked at sunlight with an even better prism. He saw 600 such dark lines in the spectrum. Then finally, Gustav Kirchhoff (1824-1887) figured out what was going on and gave the world a set of rules describing what was making both the continuous spectrum that Newton saw and the dark lines that Fraunhofer saw.

Rule #1: A luminous solid or liquid emits a continuous spectrum of all wavelengths. It has no lines in it.

Rule #2: A rarefied luminous gas emits light whose spectrum shows bright lines. These lines are called emission lines.

Rule #3: If the light from a luminous source passes through a gas, the gas may extract certain specific energies from the continuous spectrum. We then see dark lines where the energy has been removed. These dark lines are called absorption lines

What is a spectrometer?

A spectrometer is an instrument that is designed to measure how much radiation of different "colors" is reflected or emitted by an object. The results of a measurement with a spectrometer are a spectrum, a graph showing how strongly the object reflected or emitted radiation of each measured color (or wavelength.)

Let's see an example:

The vertical axis measures "percent reflectance": something that reflects more light is lighter in color than something that reflects less light. In the part of the graph labeled "visible light", one of the rocks reflects a lot more red light than it reflects blue light. It's the red sandstone--that's why it looks red to us. But the andesite and basalt don't reflect any one color of light much more than any other color, so they appear dark gray or black.

Unfortunately, the color of a rock in visible light is not a very good source of information about what kinds of minerals are present in the rock. That's why many spectrometers measure reflected or emitted radiation in the infrared part of the spectrum, in that part of the spectrum with a longer wavelength than visible light.
The graphs in the near infrared have many "dips" or "troughs" that can tell about the minerals that are present in the rock. For instance, the sharp troughs in the sandstone and limestone spectra are caused by the presence of water; these are sedimentary rocks, which formed under water. The smooth dips in the basalt and andesite spectra are caused by iron and magnesium. With clues like these, scientists can puzzle out the chemical and mineral makeup of a rock--which then become clues to the puzzle of the geologic history of the rock.



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	What Can Scientists Learn From a Spectrum? It took a long time for scientists to learn how to use a spectrum to gain insight into the universe. Isaac Newton (1642-1727) saw the spectrum from sunlight as a continuous band of colors. In 1802, William Wollaston (1766-1828) observed several dark lines in the spectrum of the Sun. He thought they represented natural divisions between the colors. Joseph von Fraunhofer (1787-1826) looked at sunlight with an even better prism. He saw 600 such dark lines in the spectrum. Then finally, Gustav Kirchhoff (1824-1887) figured out what was going on and gave the world a set of rules describing what was making both the continuous spectrum that Newton saw and the dark lines that Fraunhofer saw. Rule #1: A luminous solid or liquid emits a continuous spectrum of all wavelengths. It has no lines in it. Rule #2: A rarefied luminous gas emits light whose spectrum shows bright lines. These lines are called emission lines. Rule #3: If the light from a luminous source passes through a gas, the gas may extract certain specific energies from the continuous spectrum. We then see dark lines where the energy has been removed. These dark lines are called absorption lines What is a spectrometer? A spectrometer is an instrument that is designed to measure how much radiation of different "colors" is reflected or emitted by an object. The results of a measurement with a spectrometer are a spectrum, a graph showing how strongly the object reflected or emitted radiation of each measured color (or wavelength.) Let's see an example: The vertical axis measures "percent reflectance": something that reflects more light is lighter in color than something that reflects less light. In the part of the graph labeled "visible light", one of the rocks reflects a lot more red light than it reflects blue light. It's the red sandstone--that's why it looks red to us. But the andesite and basalt don't reflect any one color of light much more than any other color, so they appear dark gray or black. Unfortunately, the color of a rock in visible light is not a very good source of information about what kinds of minerals are present in the rock. That's why many spectrometers measure reflected or emitted radiation in the infrared part of the spectrum, in that part of the spectrum with a longer wavelength than visible light. The graphs in the near infrared have many "dips" or "troughs" that can tell about the minerals that are present in the rock. For instance, the sharp troughs in the sandstone and limestone spectra are caused by the presence of water; these are sedimentary rocks, which formed under water. The smooth dips in the basalt and andesite spectra are caused by iron and magnesium. With clues like these, scientists can puzzle out the chemical and mineral makeup of a rock--which then become clues to the puzzle of the geologic history of the rock.