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What can we learn from the different types of spectra?
This is the basic requirement of spectroscopydifferent materials emit and interact with different wavelengths (colors) of light in different ways,depending on properties such as temperature and composition. So we can use spectra, the detailed patterns of color, to figure out how hot something is and exactly what elements and compounds it's made of without ever testing it directly.
spectrum display
The first step in spectroscopy is to separate light into its color components to form a spectrum. You can do this with a glass prism, a device called a diffraction grating, or a combination of both called a grism. (Rainbows are spectra that appear naturally when sunlight falls through water droplets that act like prisms.) Spectroscopes and spectrographs are scientific tools specifically designed to capture and measure spectra.
A spectrum can be displayed as an image. However, in order to examine a spectrum in detail, to really see the subtle differences in the brightness of different colors, it is necessary to plot it on a graph. A plot of a spectrum can reveal differences in brightness and wavelength that are too subtle for the human eye to detect.
Specter of the Star Altair
A color representation of a star's spectrum, with a brightness versus wavelength plot of the same spectrum lined up directly below.
Image of a spectrum
A long horizontal rectangle has a rainbow color from blue at the left end to red at the right end. The rainbow is not continuous from left to right, but is divided by vertical black lines of varying widths. There is a series of prominent thick black lines. The distance between these lines increases from left to right.
Diagram of a spectrum
Just below the spectrum image is a plot of the same spectrum, showing brightness on the vertical y-axis versus wavelength on the horizontal x-axis. The image and graphic are aligned vertically so the relationship is clear.
Diagramming machines
The y-axis is labeled "Brightness" with an upward-pointing arrow to indicate that brightness increases from bottom to top. There are no numbers or marks on the y-axis. A label pointing to the axis and reading: "Luminosity (can be called Intensity, Count, Flux, Power, Absorption, Transmission, or Reflectance)."
The x-axis labeled "wavelength (nanometers)" ranges from about 380 nanometers at the leftmost origin to about 710 nanometers on the rightmost. There are marked ticks that are evenly spaced every 100 nanometers, from 400 to 700 nanometers. A label pointing to the x-axis reads: "Color (often referred to as wavelength, but can also be referred to as energy or frequency)."
graphed data
The spectrum appears as a drawn line with colored shading below the line. The color pattern corresponds to the coloring in the spectrum image above, with blue on the far right (shorter wavelengths) and red on the far left (longer wavelengths).
Left to right: The drawn line starts about two-thirds of the way up the y-axis, with a general upward trend 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 the height of the y-axis.
There are numerous steep and narrow valleys that indicate relatively low brightness that overshadows the general trend. These valleys vary in width and depth. The valleys correspond to the dark lines in the spectrum image above the chart. Broader valleys in the chart appear as broader lines in the image. Deeper valleys appear as darker lines.
Three of the most prominent valleys are labeled "Hydrogen". A label pointing to "Hydrogen" reads: "Astronomer's Interpretation: The peaks and valleys are labeled with the elements and compounds that caused them."
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.
Types of Spectrum: Continuous, Emission and Absorption
Infographic showing the relationship between the continuous spectrum of a star whose light falls on the gas, the emission spectrum of glow gas, and the absorption spectrum of that gas.
The graphic is divided into two parts. The top half shows a light source with light waves traversing a gas cloud. The lower half shows the three types of spectra in picture and diagram form.
Illustration
On the far left is a white circle that says "Continuous Light Source." A solid wavy line labeled "Light" extends to the right from the light source until it reaches a bluish, irregular, semi-transparent, cloud-like shape labeled "Gas Cloud." The wavy line representing the light is dotted after it enters the cloud and remains dotted to the right after it exits the cloud. A set of six shorter white wavy lines extends downward from the cloud.
spectra
The three spectra depicted and graphed are arranged horizontally and are clearly related to the elements in the figure above.
continuous spectrum
An arrow points down from the continuous light source to text that reads, "Continuous Spectrum: Spectrum containing all wavelengths emitted by a hot, dense light source."
Below the text is an image of a continuous spectrum in the form of a rectangular bar with the colors of the rainbow. From left to right, the colors are purple, blue, green, yellow, orange, and red.
Below the rainbow bar is a plot of brightness on the vertical y-axis versus wavelength on the horizontal x-axis. The wavelength axis is aligned with the rainbow bar. The arrows show that the brightness increases upwards and the wavelength increases to the right. A continuous curve is drawn on the chart. The curve is concave downwards, with a peak aligned with the blue part of the rainbow bar. The line is smooth.
emission spectrum
An arrow points down from the gas cloud to text that reads, "Emission Spectrum: Shows lines of colored light emitted by glowing gas."
Below the text is an image of an emission spectrum in the form of a black rectangular bar with five thin colored lines to represent the emission lines. From left to right, the lines are deep purple, purple, bluish purple, blue, and yellow. The distance between the lines increases from left to right.
Below the image of the emission lines is a plot of brightness on the vertical y-axis versus wavelength on the horizontal x-axis. The arrows show that the brightness increases upwards and the wavelength increases to the right. Five peaks are plotted in the graph. The wavelengths of the peaks correspond to the five emission lines in the image above.
absorption spectrum
An arrow points from light that has passed through the gas cloud down to text that reads, "Absorption Spectrum: Shows dark lines or gaps in light after light has passed through a gas."
Below the text is an image of an absorption spectrum in the form of a rainbow-colored rectangular bar with five thin black lines to represent the absorption lines. From left to right, the lines are in the deep purple, violet, bluish purple, blue, and yellow portions of the rainbow. The distance between the lines increases from left to right.
Below the image of the absorption lines is a plot of brightness on the vertical y-axis versus wavelength on the horizontal x-axis. The arrows show that the brightness increases upwards and the wavelength increases to the right. A curve is drawn on the chart. The curve has the same downward concave shape as the continuous spectrum, but also contains five deep, narrow valleys. The wavelengths of the valleys correspond to the five absorption lines in the image above.
The wavelengths of the black lines in the absorption spectrum image correspond to the wavelengths of the colored lines in the emission spectrum image. The wavelengths of the valleys in the absorption spectrum plot correspond to the peak wavelengths in the emission spectrum plot.
continuous spectra
Continuous spectra (blackbody curves) of stars
Continuous spectra of three stars of different color and magnitude. The spectra are plotted as curves in a plot of brightness on the vertical y-axis versus wavelength in nanometers on the horizontal x-axis.
Diagramming machines
The y-axis is labeled "Lightness" and has an arrow pointing up to indicate that lightness increases from the bottom to the top of the chart. No brands, numbers or units are marked.
The x-axis is labeled "wavelength (color) of light in nanometers" and ranges from near zero at the origin on the far left to 2000 nanometers on the far right, marked in uniform 250-nanometer increments.
Below the wavelength markers, the x-axis is labeled with bands of light corresponding to different wavelength ranges. The region to the left of 400 nanometers is labeled "ultraviolet" with an arrow pointing to the left. The range between 400 and 700 nanometers is called "visible light". The region to the right of 700 nanometers is labeled "infrared" with an arrow pointing to the right.
The visible light portion of the chart 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 general shape of a left bell curve. The three curves are nested and do not intersect.
Blue Star
The highest curve with the brightest wavelengths has a light blue sphere labeled "Blue Star"; 8000K; Spectral Type: A.” The blue sphere is shown at the apex of the curve at about 250 nanometers in the ultraviolet portion of the spectrum.
Red Star
The shortest curve with the faintest wavelengths has a bright red sphere labeled "Red Star"; 3000K; Spectral type: M.” The red sphere is shown at the peak of the curve at about 900 nanometers in the infrared part of the spectrum.
Yellow Star
The center curve, showing intermediate light wavelengths, has a light yellow circular sphere labeled “Yellow Star; 5000K; Spectral type: G.” The yellow sphere is shown at the apex of the curve at about 550 nanometers in the yellow part of the spectrum.
The first spectrum type to consider is thecontinuous spectrum.A continuous spectrum, as you might have guessed, is continuous. Brightness varies fairly evenly from color to color, and there's no shortage of color in an ideal continuous spectrum.
A blackbody curve is a type of continuous spectrum. This is the band of color that an object such as a star, planet, or filament emits simply because of its surface temperature.
Blackbody spectra are useful because the shape of the curve and the maximum wavelength (i.e. brightest color) are directly related to the 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.
It is also useful to understand the continuous spectrum as it can be the starting point for other types of spectra.
absorption spectrum
Andabsorption spectrumIt looks like a continuous spectrum, but some colors are noticeably darker than others or almost non-existent. These missing colors appear as black lines known asabsorption lines. As you might have guessed, absorption lines are caused by absorption: when starlight passes through a material, say a dense gas, the atoms and molecules in the gas absorb some wavelengths.
What's really interesting and very useful is that each element of the gas absorbs a very specific pattern of wavelengths. If you recognize the "signature" of that element or compound, you know it's present in the gas. The relative strength of the absorption lines (how dark they are) gives you an idea of the varying 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 furtherin a little.)
the solar spectrum
Remember when we said stars emit a continuous spectrum? Well that's notExactlyTRUE. If you look closely at the sun's spectrum, you will see that it has many absorption lines corresponding to all of the sun's elements. Some wavelengths of light produced by the sun are absorbed by atoms in the cooler layers of the sun as they travel into space travel. We know what the sun is made of from its absorption spectrum. (In fact, the second most abundant element in the universe, helium, was not discovered on Earth but as a mysterious set of absorption lines in the Sun.)
transmission spectra
Atransmission spectrumis kind ofabsorption spectrum. For example, when starlight penetrates a planet's atmosphere, some of the light is absorbed by the atmosphere and some is transmitted through it. Dark lines and faint bands of light in a transmission spectrum correspond to atoms and molecules in the planet's atmosphere. The amount of light transmitted also depends on how dense and warm the atmosphere is.
Transmission spectrum of an earth-like atmosphere
The chart titled "Transmission Spectrum of an Earth-Like Atmosphere" contains a detailed line plot of blocked light on the vertical y-axis versus wavelength on the horizontal x-axis. A satellite image of the earth is included in the background.
Diagramming machines
On the y-axis labeled "Light Blocked" is an arrow pointing up. The bottom of the arrow is labeled "Less" and the top of the arrow is labeled "More." There are no numbers, units, or markers on the y-axis.
The x-axis, labeled "wavelength (nanometers)" increases wavelength from left to right, with evenly spaced ticks at 5000; 10,000; 15,000; and 20,000 nanometers. The leftmost origin is unlabeled.
graphed data
The spectrum consists of data points connected by straight lines. The general shape is irregular, with numerous spines of varying height and width, but with no apparent systematic pattern. Some of the narrow peaks overlap wider peaks. The baseline is flat. (There is no general trend from the left to the right side of the chart.)
The prominent peaks are labeled:
- Ozone with a short peak near the origin and a slightly higher peak at around 9500 nanometers
- carbon dioxide with a mid-level peak at 2500; a high peak at 4,500 nm; and a slightly larger, very broad peak centered at 15,000 nanometers
- Methane with very short peaks of about 3500 nanometers and 7500 nanometers
- Water in a very broad short group of peaks from about 5,500 to 7,500 nanometers and a very short group of peaks between 17,500 and 20,000 nanometers
emission spectra
the pattern of aemission spectrumis the inverse of an absorption spectrum. An emission spectrum is mostly dark with light colored lines known as emission lines. Emission lines also correspond to specific atoms. Each atom has a specific color pattern 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 will find out why this is so in thenext section.)
Emission spectra are particularly useful for studying hot gas clouds. The difference in brightness of the different emission lines can tell something about the temperature and density of the gas and the relative amounts of the different elements in the gas.
reflection spectra
Reflectance spectra of materials on the earth's surface
Line plots of reflectance on the vertical y-axis versus wavelength (nanometers) on the horizontal x-axis for snow, water, vegetation, and dry soil. Each line has a different pattern.
Diagramming machines
The y-axis is labeled "Reflectance" with an arrow pointing up to indicate that the amount of light reflected from 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 2500 nanometers on the right, labeled in 400-nanometer increments.
graphed data
Four spectra are plotted. Each spectrum is displayed in a different color, with the area below the spectrum being semi-transparent. Each spectrum has a different pattern. The lines intersect at numerous wavelengths.
Snow
A white line labeled "snow" shows very high reflectivity at short wavelengths to the left (short wavelengths), with overall reflectivity decreasing from left to right (with increasing wavelength). The reflectivity is very high at 400 nanometers (near the top of the plot) and relatively low at 2500 nanometers (about a tenth of the y-axis). The left-to-right decay is not smooth: the line is wavy, with noticeable bumps at around 1000 nanometers, 1500 nanometers, and 2000 nanometers. Of all the lines drawn, the snow shows the greatest reflectivity at wavelengths from 400 to 1000 nanometers.
Agua
A blue line labeled "water" shows relatively low reflectivity at all wavelengths from 400 to 2,500 nanometers. The leftmost line starts at about one-tenth of the y-axis, where it stays until it starts tapering very gradually at about 600 nanometers. The line just bottoms out at about 1,200 nanometers, where it levels off and runs just above the ground to 2,500 nanometers on the far right.
Vegetation
A green line labeled "Vegetation" starts and ends at the bottom (about a tenth of the y-axis), with a series of very prominent, highly reflective bumps in the middle. There is a small but noticeable hump at about 550 nanometers. The line rises abruptly between about 700 and 800 nanometers and forms a tall and broad hump between 800 and 1300 nanometers. The line drops steeply, falling into a valley at around 1,450 nanometers before rising again to form another shorter hump, peaking at around 1,600 to 1,800 nanometers before dropping again. The line ends with a short, wide hump between 2100 and 2400 nanometers.
dry ground
A brown line labeled "Dry Ground" starts at 400 nanometers on the far left with very low reflectance (about one-twentieth of the y-axis). Between about 450 and 1300 nanometers the line gradually increases. After that, it shows a broad hump ending at about a fifth of the y-axis, with a very steep valley at about 1950 nanometers.
Of the four materials, the floor shows the lowest short-wave reflectance on the left and the highest long-wave reflectance on the right.
Areflection spectrumshows the colors reflected from a surface. Geoscientists use reflectance spectroscopy to study rocks, soils, ocean water, polar ice caps, mineral deposits, forests, farmlands, dust storms, volcanic eruptions, and even wildlife. Planetary scientists use reflectance spectra to find out what the surfaces of planets, moons, asteroids and comets are made of. The color pattern that a material reflects depends not only on the colors it absorbs and transmits, but also on many other factors such as roughness, shape and orientation. Reflectance spectra are usually much more complicated than emission and absorption spectra and can be quite difficult to interpret.
"Spectroscopy" with the naked eye
Spectroscopy may seem far removed from everyday experience, but in fact human color vision, the ability to see materials and make inferences about things based on color, involves a fundamental form of spectroscopy.
Ground, grass and snow can be distinguished from afar because they reflect different colors. Whole milk looks thicker and "milkier" than skim milk because it absorbs and transmits light differently. Many people can tell the difference between fluorescent light, incandescent light, and natural sunlight based on subtle differences in the "quality" (i.e., color) of the light they emit.
The fundamental difference between color vision and spectroscopy is the level of detail we can distinguish. Tools like spectroscopes and spectrographs allow us to not only separate but also accurately measure the brightness and wavelengths of the hundreds to thousands of individual colors that combine to give us the overall color.
Need a refresher? Consult previous articles.
- Part 1: Introduction to spectroscopy
- Part 2: Light and Matter
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Jump ahead in the series!
- Part 4: How absorption and emission spectra work
- Part 5: Beyond temperature and composition
- Part 6: Invisible Spectroscopy
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Last update:
July 07, 2021