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What is a spectrometer?

When objects are exposed to light, some of the this light is absorbed and some reflected. Objects that you see as black absorb a lot of light and reflect very little; objects that you see as white absorb very little and reflect a lot. Things can absorb some colors of light and reflect others--so a leaf appears green because it reflects more green and blue light and absorbs the red. Some objects--like the Sun--emit their own light. The Sun looks yellow to us because it emits more yellow light than any other color. Electromagnetic radiation comes in many forms besides the visible light that we can see: gamma rays, x-rays, ultraviolet, infrared, microwaves, and radio waves. We can't see these radiations, but there are scientific instruments that can detect them. 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.