A prism making a rainbow may seem like a simple, pretty toy to look at, but it is actually one of the most important inventions in the history of physics. By breaking light up into individual wavelengths, we can learn all sorts of interesting things about the source of that light. Although modern spectrographs create much sharper images than the rainbow from a prism, they are in principle all but identical. Read on to find out what they are and what they do.
Significance
A spectrograph is a device to measure light. When we see light emitted or reflected from an object, it usually appears to be in one particular color. Sodium lights are orange, for example, and snow is white. In reality, objects often emit multiple colors of light at the same time, either in continuous ranges or in narrow bands. By analyzing what sort of light is coming off a distant object, a spectrograph can tell us a lot about what the object we are looking at is made of, how hot it is, and what might be between it and us.
Types
Spectrographs all have a lens to focus light coming in from all directions into a single beam. That beam is then shined on something that separates it into a spectrum. Early spectrographs, called spectrometers, used a glass prism to break up the light and shine it on a surface. Scientists studying the light would then look at that light for missing bands or regions of unusual brightness. They would use a ruler to calculate the approximate wavelength of each band of light. More modern spectrographs used diffraction grating and photographic paper. The diffraction grating created a more precise image than a prism, and the photographic paper allowed the scientists to create an accurate record of the light source. Current spectrographs use diffraction grating along with special light detecting cells. These detectors are able to accurately measure the brightness of each band of light.
Function
Spectrographs are mostly used in astronomy. Light travels in small packets called photons. Extremely hot molecules, such as those found in stars, release photons of light. The frequency of the photons vary depending on how hot the molecules are and what kind of molecules they are. By analyzing the spectral radiation of a star, astronomers can learn how hot it is, and how much of it is composed of hydrogen, helium or other molecules. Spectrographs can also be used to study absorption. Gases can absorb molecules passing through. A gas will only absorb photons of a certain wavelength, then scatter them or re-emit them as photons of other wavelengths. Astronomers can sometimes spot cooler gases between an observation point and a star by noticing bands of missing light from absorption.
Identification
You can see how a spectrograph works yourself with a simple piece of clear diffraction grating. A florescent bulb appears to give off white light when you look at it with the naked eye, but looking at it through a piece of diffraction grating tells quite a different story. It will separate into three images, a red one, a green one and a blue one. A similar thing happens when you look at one of the orange sodium lamps used as night lamps in most cities. It will emit two separate orange images, one a slightly different color than the other. These are the spectral lines that astronomers look for when studying a star.
Benefits
Spectrographs can be used for many other things besides studying the emission patterns of stars. One of the most useful applications of a spectrograph is studying how fast the universe is expanding. When a sound source moves quickly toward you, it seems to have a higher pitch than normal. When it moves away from you, the pitch suddenly lowers.This is called the Doppler effect. The same thing happens with light. When a star is moving away from us, the frequency of the light we perceive coming from it seems to drop slightly. By carefully studying the frequency of the spectal lines, we can see how quickly other galaxies are moving away from ours.
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