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.

The waves are identical to each other in wavelength and amplitude. Like the mechanical wave, each is two wavelengths long, with two peaks and two troughs.

A red arrow approaches the light purple box at an angle from the top left. When it hits the top of the light purple box, it changes direction and moves up toward the top right. The two parts of the arrow form a V: The angle between the arrow and the surface of the light purple box of the reflected segment is the same as for the incoming segment. Illustration is labeled “Reflection: Light can bounce.”

Visiblelightwavelength

A red arrow representing a light ray points straight-up out of the middle of the light purple box. Illustration is labeled “Emission: Light can be emitted.”

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.)

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.

When you listen to the radio, connect to a wireless network, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves and microwaves are two types of electromagnetic waves. They only differ from each other in wavelength – the distance between one wave crest to the next.

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.)

Radio waves

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.

An xyz graph shows the relationship between the strength of the electric field on the y-axis and the strength of the magnetic field on the z-axis versus distance on the x-axis. As in the graph of the mechanical wave, the y-axis is a vertical red arrow pointing upward and the x-axis is a horizontal green arrow pointing to the right. The z-axis is a blue arrow perpendicular to the xy-plane, pointing out of the page.

Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

Their size is related to their energy. The smaller the wavelength, the higher the energy. For example, a brick wall blocks the relatively larger and lower-energy wavelengths of visible light but not the smaller, more energetic x-rays. A denser material such as lead, however, can block x-rays.

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.

Electromagneticfield

A red arrow representing a light ray points straight down from the top and passes straight through the light purple box, exiting below it. The arrow does not change as it moves into, through, and out of the box. Illustration is labeled “Transmission: Light can pass through.”

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Unlike sound waves, which must travel through matter by bumping molecules into each other like dominoes (and thus can not travel through a vacuum like space), electromagnetic waves do not need molecules to travel. They can travel through air, solid objects, and even space, making them very useful for a lot of technologies.

Electromagneticwaves

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.)

Wavelength symbol

Waves in the electromagnetic spectrum vary in size from very long radio waves that are the length of buildings to very short gamma-rays that are smaller than the nucleus of an atom.

While most of this energy is invisible to us, we can see the range of wavelengths that we call light. This visible part of the electromagnetic spectrum consists of the colors that we see in a rainbow – red, orange, yellow, green, blue, indigo, and violet. Each of these colors also corresponds to a different measurable wavelength of light.

The strength of the electric and magnetic fields corresponds to each other: The peaks of the magnetic field occur at the same points on the x-axis as the peaks of the electric field.

An xy graph shows the displacement of matter on the y-axis versus distance on the x-axis. The y-axis is a vertical red arrow pointing upward. The x-axis is a horizontal green arrow pointing to the right.

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.

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.)

Electromagneticspectrum wavelength

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.

While it’s commonly said that waves are "blocked" by certain materials, the correct understanding is that wavelengths of energy are absorbed by the material. This understanding is critical to interpreting data from weather satellites because the atmosphere also absorbs some wavelengths while allowing others to pass through.

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.

electromagnetic中文

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. In each illustration, light rays are represented by solid red lines with arrows pointing in the direction of travel. Matter is represented by a semi-transparent purple box.

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.)

Light wave

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.

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.

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.”

The wave is a perfect sine curve that begins at the origin, the intersection of the x- and y-axes, where both displacement and distance equal zero. The curve is exactly two wavelengths long, with two peaks and two troughs. The peaks and troughs have the same amplitude. A horizontal line marking the distance between two troughs is labeled “wavelength.”

Two red arrows representing light rays approach the light purple box at an angle from the top left. When they enter the light purple box, the lines become dotted and then disappear inside it. Illustration is labeled “Absorption: Light can be absorbed.”

The graph consists of two perfect sine waves oriented perpendicular to each other. Both curves begin at the origin, the intersection of the three axes, where strength of the electric field, strength of the magnetic field, and distance all equal zero. The wave plotted on the xy-plane is shown in yellow and is labeled, “electric field.” The wave plotted on the xz-plane is shown in blue and is labeled, “magnetic field.” The curves intersect at the points where they cross the x-axis.

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.

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.

Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field (Fig. 1 red arrows) couples with a magnetic field (Fig.1 blue arrows).

A red arrow representing a light ray approaches the light purple box at an angle from the top left. When it enters the box, it changes direction, moving through the box at a steeper angle. When it exits the box, it changes direction again, moving at the same angle that it entered the box. Illustration is labeled “Refraction: Light can bend.”

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.