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Spectroscopy 101 – Types of Spectra and Spectroscopy

What can we learn from different types of spectra?

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

Double rainbow
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.

Picture and graph of a spectrum stacked vertically. Picture of a Spectrum (top): Rectangle with rainbow coloring from blue at left to red at right. Rainbow broken up with vertical black lines of varying width. Graph of a Spectrum (bottom): Brightness on y-axis increases from bottom to top. Callout reads, “Brightness (might be labeled as intensity, counts, flux, power, absorbance, transmittance, or reflectance).” Wavelength in nanometers on x-axis ranges from 400 to 700. Callout reads, “Color (often labeled as wavelength, but can also be labeled as energy or frequency).” Data graphed as line with area below line colored to match the blue to red of picture above. Line has many narrow valleys that correspond to the dark lines in the picture. Three valleys labeled “Hydrogen.” Arrow pointing to “Hydrogen” label reads, “Astronomer’s Interpretation: Peaks and valleys are labeled with the elements and compounds that caused them.” See Image Description for more details.
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).

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.

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.

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 with a light wave passing into, through, and out of a cloud of gas. The bottom half shows the three types of spectra in picture and graph forms. From left to right: “Continuous Spectrum” is shown as a continuous rainbow and a smooth hump-shaped line on a graph of brightness versus wavelength; “Emission Spectrum” is shown as a series of thin colored lines separated by black spaces, and a series of sharp peaks on a graph of brightness versus wavelength; and “Absorption Spectrum” is shown as a rainbow with a series of thin black lines replacing some of the colors, and a smooth curve with a series of sharp valleys on a graph of brightness versus wavelength. Select View Description for more details.
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).

Continuous Spectra

Graph of brightness versus wavelength of light, with three curves, each representing the continuous spectrum of a different color and temperature of star. The curve with the brightest wavelengths, with a peak brightness in the ultraviolet, is labeled “Blue star; 8,000 K; Spectral type: A” The curve with the dimmest wavelengths and a peak in the infrared is labeled, “Red star; 3,000 K; Spectral type: M” The curve showing wavelengths with intermediate brightness and a peak in the visible part of the spectrum is labeled “Yellow star; 5,000 K; Spectral type: G.” See the Image Description for more details.
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).

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.)

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.)

Four sets of absorption and emission spectra, stacked vertically. From top to bottom: sodium, nitrogen, hydrogen, and oxygen. Each spectrum is shown as a long horizontal rectangle with alternating sections of color and solid black (no color). The color forms a rainbow pattern from purple (400 nanometers) on the left to red (700 nanometers) on the right. The absorption and emission spectra of each element are inverses of each other: wavelengths that are black (colorless) in absorption spectrum are brightly colored in the emission spectrum, and vice versa. Each set of absorption and emission spectra (each element) has a different pattern.
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.)

Illustration of the SUn's absorption and emission spectra. The spectra of elements are labeled with letters A through G.
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.
Transmission Spectrum of an Earth-Like Atmosphere: Graph of light blocked versus wavelength of light in nanometers, for an Earth-like atmosphere. Light blocked on the y-axis increases from bottom to top. Wavelength on the x-axis ranges to 20,000 nanometers, marked in equal increments of 5,000 nanometers. The graphed line is irregular, with numerous peaks of various heights and widths. Peaks for ozone, carbon dioxide, methane, and water are labeled. Select View Description for more details.
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.

Emission Spectra

Spectra of H, He, N, O, Ar, Ne, Xe, and Hg
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.

Labeled spectrum with images of element distribution
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

Graph titled “Reflectance Spectra: Earth’s Surface Materials” compares the visible-to-near-infrared reflectance spectra of snow, water, vegetation, and dry soil. 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. Each spectrum has a unique pattern, showing that each material reflects different amounts of different wavelengths of light. Select View Description for more details.
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).

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”

Blue-green water, brown rock, grey clouds, green plants
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.

Need a refresher? Check out previous articles.

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