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Spectroscopy 101 – Light and Matter

What are light and matter and how do they interact with each other?

How is it possible to figure out such detailed information about materials on Earth and in space based only on color? Spectroscopy works because light and matter interact with each other in very specific and predictable ways. Before getting into the gory details, let’s review some relevant basics about light and matter. (If you know this already, feel free to jump ahead.)

Spectroscopy works because light and matter interact with each other in very specific and predictable ways.

Light

Gamma rays, X-rays, ultraviolet light, visible light (the visible rainbow), infrared light, microwaves, and radio waves are all forms of light, also called electromagnetic radiation. Together, they make up the electromagnetic spectrum. (That’s right, the radio waves that carry music from the station to your radio, the microwaves that heat up your food, and the X-rays dentists use to detect tooth decay are all forms of light.)

Light has a number of fascinating (and somewhat odd) characteristics that are important for spectroscopy.

Graphic titled “The Electromagnetic Spectrum” shows the shortest, most energetic wavelengths at the left and the longest, least energetic to the right, illustrated as a long, partially colored strip that ends in arrows. From left to right, the types of light are paired with their measurements: gamma ray (0.000001 to 0.01 nanometers), X-ray (0.01 to 10 nanometers), ultraviolet (10 to 380 nanometers), visible light (380 to 780 nanometers), infrared (780 nanometers to 0.3 millimeters), microwave (0.3 millimeters to 10 centimeters), radio (10 centimeters to 10 meters). On the graphic, visible light is represented by a rainbow of colors: violet, indigo, blue, green, yellow, orange, and red. The background is largely purple.
Gamma rays, X-rays, ultraviolet light, visible light (the visible rainbow), infrared light, microwaves, and radio waves are all forms of light, also called electromagnetic radiation. Together, they make up the electromagnetic spectrum. Each band of light has a different range of wavelengths: Gamma rays are the shortest and radio waves are the longest. Get the full electromagnetic spectrum infographic. 
Credit: NASA, ESA, L. Hustak (STScI).

Waves

Light behaves like a wave. You are probably familiar with waves: water waves that ripple across a pond, sound waves that vibrate air and ear drums, and seismic (earthquake) waves that cause the ground to shake. These are all mechanical waves—energy that propagates through matter, causing it to move up and down, back and forth, or side to side.

Light waves are similar, but while mechanical waves cause oscillations in matter, light waves consist of electric and magnetic fields oscillating perpendicular to each other. Mechanical waves need matter in order to propagate, but light waves can travel through completely empty space as well as through matter. (If the idea of oscillating electric and magnetic fields does not make much sense, don’t worry. You don’t really need to know too much about it.)

A graph of a mechanical wave showing a sine curve plotted on an xy graph. Select View Description for more details.
A mechanical wave is a propagation of energy through matter. The wavelength is the distance between two consecutive peaks (the highest part of the wave) or troughs (the lowest part of the wave). On this graph, the x-axis is distance and the y-axis is the displacement of matter. Get the full mechanical wave diagram.
Credit: NASA, ESA, L. Hustak (STScI).
A graph of an electromagnetic wave showing two identical sine curves, one plotted on the xy plane labeled “electric field” and the other plotted on the xz plane labeled “magnetic field.” Select View Description for more details.
Light waves involve oscillations of electric and magnetic fields rather than oscillations of matter. The wavelength is its color. On this graph the x-axis is distance; the y-axis is the strengh of the electric field; and the z-axis is the strength of the magnetic field. Get the full electromagnetic wave diagram.
Credit: NASA, ESA, L. Hustak (STScI).

One way to measure waves is by their wavelength. Wavelength is the distance between successive peaks. The wavelength of a light wave is the distance between peaks in the electric and magnetic fields.

While that definition might make it sound like wavelength is a property that only a physicist could appreciate, most people are actually very familiar with the concept of wavelength: Human eyes recognize differences in wavelength as differences in color. On the visible part of the spectrum, shorter wavelengths look bluer and longer wavelengths look redder.

From the left side of the image, a thick white line representing a ray of light labeled “Sunlight” enters a translucent pyramid-shaped prism. When the white ray of light enters the prism, it spreads out and forms a rainbow, with red on the top and purple at the bottom. When the rainbow exits the prism, it spreads out even more, and the colors of the rainbow are more prominent. From top to bottom they are red, orange, yellow, green, blue, and purple. On the right, the rainbow is labeled, “Visible light.” Above the red end of the rainbow with an arrow pointing up is the label “Infrared.” Below the purple end of the rainbow with an arrow pointing down is the label “Ultraviolet.”
Sunlight that passes through a prism naturally separates into its component colors in a very specific order—rainbow order—based on wavelength. This rainbow is known as the visible spectrum. In addition to visible light, sunlight also contains significant amounts of infrared and ultraviolet light, both of which are invisible to human eyes. Get the full prism illustration.
Credit: NASA, ESA, L. Hustak (STScI).

Wavelength is also what differentiates the various bands of light on the electromagnetic spectrum. When people talk about different “types” of light, they are referring to broad differences in wavelength. Gamma rays have the shortest wavelengths and radio waves have the longest. Visible light is in the middle. You can think of gamma rays, X-rays, ultraviolet light, infrared light, microwaves, and radio waves as bands of invisible color.

Human eyes recognize differences in wavelength as differences in color.

Particles

Light also behaves like a particle. A particle of light is called a photon. Each individual photon has a very specific amount of energy (no more, no less), which corresponds to its wavelength. Blue photons carry more energy than red photons. Ultraviolet photons carry more energy than infrared photons. Sometimes photons are described as “packets of energy.”

So, you can think of light as waves or you can think of it as streams of photons. Astronomers use both terms, depending on what they are trying to study or explain. (If you are confused as to how light can be both a particle and a wave at the same time, don’t despair. You are not alone.)

The important thing to keep in mind when it comes to spectroscopy is that wavelength and energy are effectively the same thing. Human eyes detect differences in wavelength and energy as differences in color.

A horizontal bar with rainbow coloring, from violet (left) to red (right). The colors grade into one another, with the far-left and far-right ends fading into the dark background of the graphic. The center region of the rainbow is labeled “Visible Light.” The region left of violet is labeled “Ultraviolet.” The region right of red is labeled “Infrared.” Running left to right through the middle of the rainbow bar is a squiggly sine wave. The distance between wave peaks increases (the waves get longer) from violet at the left to red at the right. Seven white dashed, vertical lines are spaced in equal increments across the rainbow. The leftmost line is in the violet region and is labeled “400 nanometers.” The rightmost line, in the bright red region, is labeled “700 nanometers.” Along the bottom is an arrow pointing from violet at the left to red at the right. The violet end, at the left, is labeled “higher energy (shorter wavelength).” The red end, at the right, is labeled “lower energy (longer wavelength).”
Different colors of light have different wavelengths and energy. Light toward the red end of the spectrum has longer wavelengths and lower energy than light toward the violet end of the spectrum. Infrared light has even longer wavelengths than red; while ultraviolet has even shorter wavelengths than violet. Wavelength is described in units of length. Ultraviolet, visible, and infrared light are typically given in microns (µm), nanometers (nm), or ångstroms (Å). (1 centimeter = 10,000 microns, 10 million nanometers, or 100 million ångstroms.) Energy is measured in electron volts (eV). Get the full color, wavelength, and energy comparison image.
Credit: NASA, ESA, L. Hustak (STScI).

Matter

Matter is the scientific catch-all word for stuff—anything that has mass and takes up space. Matter is made of microscopic particles called atoms. Atoms are made of even smaller, or subatomic, particles known as protons, neutrons, and electrons. Atoms can combine to form molecules.

Solids, liquids, and gases are all forms of matter. Planets, stars, nebulae, and galaxies are all made of matter. Rocks, water, air, dust bunnies, giraffes, viruses, spinach, coffee cups, and cowboy boots are all made of matter.

Diamond crystal; diamond molecular structure; carbon atom
Matter is made of atoms, which are made of protons, neutrons, and electrons. Diamond, for example, is made of carbon atoms bonded together in a repeating pattern. Most carbon atoms consist of 6 protons and 6 neutrons in the nucleus, and 6 electrons, each at a specific energy level outside the nucleus.
Credit: NASA, ESA, L. Hustak (STScI).

Protons and neutrons make up the nucleus of an atom, while electrons reside outside the nucleus. Although it’s actually much more complicated than this, you can think of each electron as occupying a particular energy level (sometimes referred to as an “orbital” or “shell”) around the nucleus. Electrons are a bit bizarre in that they can “jump” or “drop” (transition) from one energy level to another, but they can’t exist between two levels. (Why this is important? Keep reading or skip ahead.)

Interactions between Light and Matter

As you may have gathered, light and matter are intricately linked. 

Matter gives off light. Every object emits, or gives off, light of one sort or another simply because of its temperature. Glowing objects like stars, galaxies, light bulbs, and lava are all sources of visible light. Cooler objects like planets, dust grains, rocks, trees, animals, and icebergs don’t glow in visible light, but they do emit significant amounts of infrared light. Matter can also give off very specific colors of light depending on what it is made of and how it is interacting with other forms of matter and energy.

Graphic titled “Behaviors of Light” with five simple illustrations showing how light rays interact with matter. From left to right: absorption, emission, transmission, reflection, and refraction. Light rays are shown as red lines with arrows at one end. Matter is represented by a semi-transparent purple box. Absorption – Light can be absorbed: Red arrows disappear into purple box. Emission – Light can be emitted: Arrow points up out of purple box. Transmission – Light can pass through: Arrow travels through purple box. Reflection – Light can bounce: Arrow bounces off purple box. Refraction – Light can bend: Arrow is bent as it moves into and out of purple box. Click View Description for more details.
Light and matter interact with each other. Matter can absorb light (take it in); emit light (give it off); transmit light (allow it to pass through); reflect light (cause it to bounce off); and refract light (cause it to change speed and direction). Get the full Behaviors of Light diagram.
Credit: NASA, ESA, L. Hustak (STScI).

Light interacts with matter. When light encounters matter, a lot of things can happen. A few are particularly important to keep in mind when it comes to spectroscopy:

  • Some light is absorbed and transformed into other forms of energy. Asphalt is black because it absorbs all colors of visible light very well. It heats up quickly in direct sun because a lot of that light is transformed into thermal energy (which is then emitted back out as invisible infrared light). Plants absorb mostly red and blue wavelengths of sunlight and turn them into chemical energy to live and grow.
  • Light that is not absorbed by matter can be reflected off it. Snow is white because it reflects all colors of visible light extremely well. Grass is green because it reflects a lot of green wavelengths of sunlight.
  • Light that is not absorbed or reflected by matter can be transmitted through it. Window glass is transparent, or “see-through,” because it transmits all colors of visible light. Strawberry jello is red because it transmits red light and absorbs all other colors.

Materials on Earth and in space are continuously emitting and interacting with light. What makes one material look different from another is the way that different wavelengths of light interact with it.

Need a refresher? Check out previous articles.

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