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Spectroscopy 101 – Types of Spectra and Spectroscopy [1]

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Date: 2025-02

The basic premise of spectroscopy is that different materials emit and interact with different wavelengths (colors) of light in different ways, depending on properties like temperature and composition. We can therefore use spectra—the detailed patterns of colors—to figure out things like exactly how hot something is and exactly what elements and compounds it is made of, without ever sampling it directly.

Visualizing Spectra

Rainbow over Waimea Canyon State Park, Hawaii. Rainbows are spectra that form naturally when sunlight refracts and spreads out as it passes through water droplets. Credit: E. Marcucci.

The first step in spectroscopy is separating light into its component colors to make a spectrum. You can do this using a glass prism, a device called a diffraction grating, or a combination of the two, known as a grism. (Rainbows are spectra that appear naturally when sunlight passes through water droplets, which act like prisms.) Spectroscopes and spectrographs are scientific tools designed specifically for capturing and measuring spectra.

A spectrum can be displayed as an image. However, in order to study a spectrum in detail—to really see the subtle differences in brightness of different colors—it needs to be plotted on a graph. A graph of a spectrum can reveal differences in brightness and wavelength that are too subtle for human eyes to detect.

A spectrum shows details in the brightness of different colors that are not visble to the naked eye. Detectors in a telescope can measure the precise brightness of individual wavelengths. Those data can be plotted on a graph of brightness vs. wavelength. This spectrum is from the bright star Altair. Note: There are a number of different ways to plot spectra. Wavelength can increase from left to right, or from right to left. Brightness can increase from bottom to top (so emission lines are peaks and absorption lines are valleys) or from top to bottom (with emission lines as valleys and absorption lines as peaks). On some graphs, units are on a linear scale (1, 2, 3 . . ), on others they are on a log scale (1,10,100). Get the full spectrum of Altair. Credit: NASA, ESA, and L. Hustak (STScI). Spectrum of the Star Altair A color illustration of a star’s spectrum with a brightness versus wavelength graph of the same spectrum aligned directly below. Picture of a Spectrum A long horizontal rectangle has a rainbow coloring from blue on the far left to red on the far right. The rainbow is not continuous from left to right, but is instead broken up with vertical black lines of varying width. There is a series of prominent, thick black lines. The spacing between these lines increases from left to right. Graph of a Spectrum Directly below the picture of the spectrum is a graph of the same spectrum showing brightness on the vertical y-axis versus wavelength on the horizontal x-axis. The picture and graph are aligned vertically so that the relationship is clear. Graph Axes The y-axis is labeled “Brightness” with an arrow pointing up to indicate that brightness increases from bottom to top. There are no numbers or tick marks on the y-axis. A label pointing to the y-axis reads, “Brightness (might be labeled as intensity, counts, flux, power, absorbance, transmittance, or reflectance).” The x-axis labeled “Wavelength (nanometers)” ranges from about 380 nanometers at the origin on the far left to about 710 nanometers on the far right. There are evenly spaced, labeled tick marks every 100 nanometers from 400 to 700 nanometers. A label pointing to the x-axis reads, “Color (often labeled as wavelength, but can also be labeled as energy or frequency).” Graphed Data The spectrum appears as a graphed line with colored shading below the line. The color pattern matches the coloring of the picture of the spectrum above, with blue at the far right (shortest wavelengths) and red at the far left (longest wavelengths). From left to right: The graphed line begins about two-thirds of the way up the y-axis, with a general trend upwards showing increasing brightness from 380 nanometers to a peak at about 410 nanometers. The line then shows a gradual decrease in brightness from 410 nanometers to 710 nanometers, ending at a point about one-third of the way up the y-axis There are numerous steep, narrow valleys indicating relatively low brightness superimposed on the general trend. These valleys vary in width and depth. The valleys correspond to the dark lines on the picture of the spectrum above the graph. Wider valleys on the graph appear as wider lines in the picture. Deeper valleys appear as darker lines. Three of the most prominent valleys are labeled “Hydrogen.” A label pointing to “Hydrogen” reads, “Astronomer’s Interpretation: Peaks and valleys are labeled with the elements and compounds that caused them.”

The basic premise of spectroscopy is that different materials emit and interact with different wavelengths (colors) of light in different ways, depending on properties like temperature and composition.

Types of Spectra

All spectra show basically the same thing: how brightness varies with wavelength. Scientists often classify spectra based on the key light-matter interactions they represent and how they are used.

Stars emit light, which travels out in all directions and interacts with other materials in space. The broad range of colors that a star emits depends on its temperature. When starlight passes through a cloud of gas, some of the light is absorbed and some is transmitted through the gas. Starlight can also heat up a cloud of gas, exciting the atoms and molecules within the gas, and causing it to emit light. The spectrum of light that a cloud of gas emits depends on its temperature, density, and composition. Get the full Types of Spectra infographic. Credit: NASA, ESA, and L. Hustak (STScI). Types of Spectra: Continuous, Emission, and Absorption Infographic showing the relationship between the continuous spectrum of a star whose light is shining on gas, the emission spectrum of glowing gas, and the absorption spectrum of that gas. The graphic is divided into two parts. The top half shows a light source, a light wave, and a cloud of gas. The bottom half shows the three types of spectra in picture and graph forms. Illustration On the far left is a white circle labeled “Continuous light source.” A solid wavy line labeled “light” extends to the right from the light source until it reaches a blueish irregular semi-transparent cloud-like form labeled “Cloud of gas.” The wavy line representing light is dashed after it enters the cloud and remains dashed to the right after it leaves the cloud. A set of six shorter, white wavy lines extends down from the cloud. Spectra Three spectra shown in picture and graph forms are arranged horizontally, and are related to the elements of the illustration above. Continuous Spectrum An arrow points down from the continuous light source to text that reads “Continuous Spectrum: Spectrum that contains all wavelengths emitted by a hot, dense, light source.” Below the text is a picture of a continuous spectrum in the form of a rainbow-colored rectangular bar. From left to right, the colors are purple, blue, green, yellow, orange, and red. Below the rainbow bar is a graph of “Brightness” on the vertical y-axis versus “Wavelength” on the horizontal x-axis. The “Wavelength” axis is aligned with the rainbow bar. Arrows show that brightness increases upward and wavelength increases toward the right. A continuous curve is plotted on the graph. The curve is concave down, with a peak that aligns with the blue part of the rainbow bar above. Emission Spectrum An arrow points down from the gas cloud to text that reads “Emission Spectrum: Shows colored lines of light emitted by glowing gas.” Below the text is a picture of an emission spectrum in the form of a black rectangular bar with five thin colored lines to represent emission lines. From left to right, the lines are dark purple, purple, blueish purple, blue, and yellow. The distance between lines increases from left to right. Below the bar is a graph of “Brightness” on the vertical y-axis versus “Wavelength” on the horizontal x-axis. Arrows show that brightness increases upward and wavelength increases toward the right. Five peaks are plotted on the graph. The wavelengths of the peaks correspond to the five emission lines in the spectrum bar above. Absorption Spectrum An arrow points down from light that has passed through the gas cloud to text that reads “Absorption Spectrum: Shows dark lines or gaps in light after the light passes through a gas.” Below the text is a picture of an absorption spectrum in the form of a rainbow-colored rectangular bar with five thin black lines to represent absorption lines. From left to right, the lines are in the dark purple, purple, blueish purple, blue, and yellow parts of the rainbow. The distance between lines increases from left to right. Below the bar is a graph of “Brightness” on the vertical y-axis versus “Wavelength” on the horizontal x-axis. Arrows show that brightness increases upward and wavelength increases toward the right. A curve is plotted on the graph. The curve has the same concave-down shape as the “Continuous Spectrum,” but also includes five deep, narrow valleys. The wavelengths of the valleys correspond to the five absorption lines in the spectrum bar above. The wavelengths of the black lines in the bar of the “Absorption Spectrum” correspond to the wavelengths of colored lines in the bar of the “Emission Spectrum.” The wavelengths of the valleys in the graph of the “Absorption Spectrum” correspond to the wavelengths of colored lines in the peaks of the “Emission Spectrum” graph.

Continuous Spectra

A blackbody curve is a type of continuous spectrum that is directly related to the temperature of an object. A star with a temperature of 8,000 kelvins (roughly 8,000 degrees Celsius or 14,000 degrees Fahrenheit) is brighter and looks bluer than a star that is 3,000 K (2,700°C or 5,000°F ) which is dimmer and redder. You can use this type of spectrum to calculate the temperature of an object. (Although stars are not perfect blackbodies, the blackbody curve describes the shape of a star’s overall spectrum quite well.) Get the full Continuous Spectra infographic. Credit: NASA, ESA, L. Hustak and A. James (STScI). Continuous Spectra (Blackbody Curves) of Stars Continuous spectra of three stars of different color and brightness. The spectra are plotted as curves on a graph of brightness on the vertical y-axis versus wavelength in nanometers on the horizontal x-axis. Graph Axes The y-axis is labeled “Brightness of Light” and has an arrow pointing upward to indicate that brightness increases from the bottom to the top of the graph. There are no tick marks, numbers, or units labeled. The x-axis is labeled “Wavelength (Color) of Light in Nanometers” and ranges from almost approaching zero at the origin on the far left to 2,000 nanometers on the far right, marked in even increments of 250 nanometers. Below the wavelength marks, the x-axis is labeled with bands of light that correspond to different wavelength ranges. The region to the left of 400 nanometers is labeled “Ultraviolet” with an arrow pointing to the left. The region between 400 and 700 nanometers is labeled “Visible light.” The region to the right of 700 nanometers is labeled “Infrared” with an arrow pointing to the right. The visible light region of the graph between 400 and 700 nanometers has a rainbow-colored background, with purple at 400 and red at 700. Graphed Curves Three spectra are plotted. Each spectrum has the overall shape of a bell curve skewed to the left. The three curves are nested one inside the other and do not intersect. Blue Star The tallest curve, with the brightest wavelengths, has a glowing blue sphere labeled, “Blue star; 8,000 K; Spectral type: A.” The blue sphere is plotted at the peak of the curve at about 250 nanometers, in the ultraviolet part of the spectrum. Red Star The shortest curve, with the dimmest wavelengths, has a glowing red sphere labeled, “Red star; 3,000 K; Spectral type: M.” The red sphere is plotted at the peak of the curve at about 900 nanometers, in the infrared part of the spectrum. Yellow Star The curve in the middle, showing wavelengths with intermediate brightness, has a glowing yellow circle sphere labeled, “Yellow star; 5,000 K; Spectral type: G.” The yellow sphere is plotted at the peak of the curve at about 550 nanometers, in the yellow part of the spectrum.

The first type of spectrum to consider is the continuous spectrum. A continuous spectrum is, as you might guess, continuous. The brightness varies fairly evenly from color to color, and in an ideal continuous spectrum, there are no missing colors.

A blackbody curve is one type of continuous spectrum. This is the band of colors that an object like a star, planet, or light bulb filament emits based simply on its surface temperature.

Blackbody spectra are useful because the shape of the curve and the peak wavelength (i.e., the brightest color) are directly related to surface temperature and nothing else. Hot stars emit more blue than red light, and therefore appear bluer in the night sky. Cool stars emit more red than blue light, and appear redder.

The continuous spectrum is also useful to understand because it can be the starting point for other types of spectra.

The continuous spectrum can be the starting point for other types of spectra.

Absorption Spectra

An absorption spectrum looks like a continuous spectrum, but with some colors significantly dimmer than others, or nearly missing. These missing colors appear as black lines known as absorption lines. As you might have guessed, absorption lines are caused by absorption: When starlight passes through a material—say a dense gas—atoms and molecules in the gas absorb some wavelengths.

What is really interesting and very useful is that each element in the gas absorbs a very specific pattern of wavelengths. If you recognize the “signature” of that element or compound, you know it exists in the gas. The relative strengths of the absorption lines (how dark they are) gives you an idea of the different amounts of each material and the temperature and density of the gas. (Why does each element have a specific signature? It has to do with those electrons moving between energy levels, which we explain more in a bit.)

Simplified illustration of absorption and emission spectra. Every element has a unique set of absorption and emission lines, or spectral signature. The absorption and emission spectra of

each element are inverses of each other. The wavelengths of a particular element’s absorption lines are the same as the wavelengths of its emission lines. Get the full Absorption and Emission Spectra diagram. Credit: NASA, ESA, and L. Hustak (STScI).

The Solar Spectrum Remember when we said that stars emit a continuous spectrum? Well, that’s not precisely true. If you look very carefully at the spectrum of the Sun, you will see that it has a lot of absorption lines, all of which correspond to elements in the Sun. Some wavelengths of light generated by the Sun get absorbed by atoms in cooler layers of the Sun as they travel out toward space. We know what the Sun is made of because of its absorption spectrum. (In fact, the second-most common element in the universe—helium—was discovered not on Earth, but as a mysterious set of absorption lines in the Sun.) Spectrum of the Sun published by astronomer and Catholic priest Angelo Secchi in 1877. Secchi was one of the first astronomers to characterize stars based on their spectra. Credit: P.A. Secchi, Le Stelle: Saggio di Astronomia Siderale, Milan, 1877.

Transmission Spectra

A transmission spectrum is a type of absorption spectrum. When starlight passes through the atmosphere of a planet, for example, some of the light is absorbed by the atmosphere and some is transmitted through it. The dark lines and dim bands of light in a transmission spectrum correspond to atoms and molecules in the planet’s atmosphere. The amount of light that is transmitted also depends on how dense the atmosphere is and how warm it is. Video: How Do We Learn About a Planet’s Atmosphere? Learn how Webb will use transmission spectroscopy to study the atmospheres of exoplanets.

A transmission spectrum of and Earth-like atmosphere shows wavelengths of sunlight that molecules like ozone, water, carbon dioxide, and methane absorb. Molecules tend to have wide absorption bands rather than narrow absorption lines. Transmission spectroscopy is used to study the atmospheres of planets orbiting distant stars. Notice that on this graph, the y-axis shows amount of light blocked rather than brightness, so brightness decreases from bottom to top. Get the full transmission spectrum graph. Credit: NASA, ESA, and L. Hustak (STScI). Model transmission spectrum from Lisa Kaltenegger and Zifan Lin 2021 ApJL 909 L2. Transmission Spectrum of an Earth-Like Atmosphere Graphic titled “Transmission Spectrum of an Earth-Like Atmosphere” includes a detailed line graph of light blocked on the vertical y-axis versus wavelength on the horizontal x-axis. A satellite image of Earth is included in the background. Graph Axes The y-axis, labeled “Light blocked,” has an arrow pointing upward. The bottom of the arrow is labeled “Less,” and the top of the arrow is labeled “More.” There are no numbers, units, or tick marks on the y-axis. The x-axis, labeled “Wavelength (nanometers),” increases in wavelength from left to right, with evenly spaced tick marks at 5,000; 10,000; 15,000; and 20,000 nanometers. The origin at the far left is not labeled. Graphed Data The spectrum consists of data points connected by straight lines. The overall shape is irregular, with numerous peaks of various heights and widths, but no obvious systematic pattern. Some of the narrow peaks are superimposed on broader peaks. The baseline is flat. (There is no overall trend from the left to right side of the graph.) Prominent peaks are labeled: Ozone with a short peak near the origin and a somewhat taller peak at about 9,500 nanometers

Carbon dioxide with a medium-height peak at 2,500; a tall peak at 4,500 nanometers; and a slightly taller and very broad peak centered at 15,000 nanometers

Methane with very short peaks at about 3,500 nanometers and 7,500 nanometers

Water in a very broad short set of peaks from about 5,500 to 7,500 nanometers, and a set of very short peaks between 17,500 and 20,000 nanometers

Emission Spectra

Photograph of the emission spectra of gases measured in a laboratory. In the 1850s, scientists discovered that different elements emit different patterns of light when heated in a flame. They noticed that the patterns of known elements studied in the lab correspond to patterns seen in the absorption lines in the Sun. Credit: M. Richmond, RIT.

The pattern of an emission spectrum is the inverse of an absorption spectrum. An emission spectrum is mostly dark with bright lines of color known as emission lines. Emission lines also correspond to specific atoms. Each atom has a specific pattern of colors that it emits. In fact, the wavelengths of an atom’s emission lines are exactly the same as the wavelengths of its absorption lines. (We’ll get to why this is in the next section.)

Emission spectra are particularly useful for studying clouds of hot gas. The difference in brightness of different emission lines can tell you something about the temperature and density of the gas and the relative amounts of different elements in the gas.

Emission spectrum of the Southern Crab Nebula. Bright emission lines show that the hot gas in the center of the Southern Crab Nebula contains oxygen, hydrogen, nitrogen, and sulfur. Credit: NASA, ESA, and J. DePasquale (STScI).

Reflectance Spectra

Reflectance spectra of common materials on Earth’s surface. Different materials reflect different amounts of various colors of light. (Most of the colors we see are the reflected colors.) Scientists can compare the reflectance spectrum of a distant planet to reflectance spectra of different materials on Earth’s surface to figure out what rocks, minerals, liquids, and ices could be on the planet. Get the full reflectance spectra graph. Credit: NASA, ESA, and L. Hustak (STScI). Reflectance Spectra of Materials on Earth’s Surface Graph titled “Reflectance Spectra: Earth’s Surface Materials” compares the visible-to-near-infrared reflectance spectra of snow, water, vegetation, and dry soil. Spectra are plotted as lines on the graphs of Reflectance on the vertical y-axis versus Wavelength (nanometers) on the horizontal x-axis. Each line has a different pattern. Graph Axes The y-axis is labeled “Reflectance” with an arrow pointing up to indicate that the amount of light reflected by the material increases from bottom to top. The x-axis is labeled “Wavelength (nanometers)” and ranges from 400 nanometers at the origin on the left to 2,500 nanometers on the right, labeled in increments of 400 nanometers. Graphed Data Four spectra are graphed. Each spectrum is shown in a different color, with the area below the spectrum filled with a semi-transparent color. Each spectrum has a different pattern. The lines intersect with one another at numerous wavelengths. Snow A white line labeled “Snow” shows very high reflectance of short wavelengths on the left, with a general decrease in reflectance with increasing wavelengths from left to right. Reflectance is very high at 400 nanometers (nearly at the top of the graph) and relatively low at 2,500 nanometers (about one-tenth of the way up the y-axis). The decrease from left to right is not even: The line is wavy, with noticeable humps at about 1,000 nanometers; 1,500 nanometers; and 2,000 nanometers. Of all of the lines plotted, snow shows the highest reflectivity at wavelengths of 400 to 1,000 nanometers. Water A blue line labeled “Water” shows relatively low reflectance at all wavelengths from 400 to 2,500 nanometers. The line begins at the far left about one-tenth of the way up the y-axis, where it remains until it begins to drop off very gradually at about 600 nanometers. The line reaches a low point just at about 1,200 nanometers, where it flattens out, running just above the bottom to 2,500 nanometers at the far right. Vegetation A green line labeled “Vegetation” begins and ends low (about one-tenth of the way up the y-axis), with a number of very prominent humps of high reflectivity in between. There is a small, but noticeable hump at about 550 nanometers. The line rises steeply between about 700 and 800 nanometers, forming a high broad hump between 800 and 1,300 nanometers. The line drops steeply, forming a valley at about 1,450 nanometers, before rising again to form another, shorter hump that peaks between about 1,600 and 1,800 nanometers before falling again. The line ends with a short, broad hump between 2,100 and 2,400 nanometers. Dry Soil A brown line labeled “Dry soil” begins with very low reflectivity (about one-twentieth of the way up the y-axis) at 400 nanometers on the far left. The line rises gradually between about 450 and 1,300 nanometers. From there, it shows a broad hump ending about one-fifth of the way up the y-axis, with a very sharp valley at about 1,950 nanometers. Of all four materials, dry soil shows the lowest reflectivity at short wavelengths on the left, and the highest reflectivity at long wavelengths on the right.

A reflectance spectrum shows the colors that reflect off a surface. Earth scientists use reflectance spectroscopy to study rocks, soil, ocean water, ice caps, mineral deposits, forests, farmland, dust storms, volcanic eruptions, and even wildlife. Planetary scientists use reflectance spectra to figure out what the surfaces of planets, moons, asteroids, and comets are made of. The pattern of colors that a material reflects depends on not only what colors it is absorbing and transmitting, but also many other factors, like roughness, shape, and orientation. Reflectance spectra are typically a lot more complicated than emission and absorption spectra, and can be quite difficult to interpret.

Naked-Eye “Spectroscopy”

Cliffs along the coast of Cornwall, England. It’s possible to distinguish between water, rock, clouds, and vegetation from a distance or in a photograph because they absorb, reflect, refract, and transmit light in different ways. Credit: M.W. Carruthers.

Spectroscopy may seem remote from everyday experience, but in fact, human color vision—the ability to recognize materials and make inferences about things based on color—involves a basic form of spectroscopy.

It’s possible to tell the difference between soil, grass, and snow from a distance because they reflect different colors. Whole milk looks thicker and “milkier” than skim milk because of differences in the way they absorb and transmit light. Many people can tell the difference between fluorescent lights, incandescent lights, and natural sunlight based on subtle differences in the “quality” (i.e., color) of the light they emit.

Most people rely on the basic principle of spectroscopy—that color carries information—every day without even knowing it.

The basic difference between color vision and spectroscopy is the level of detail that we can make out. Tools like spectroscopes and spectrographs allow us not only to separate, but also to precisely measure the brightness and wavelengths of hundreds to thousands of individual colors that combine to give us the overall color.

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