Spectral line

A spectral line is a special pattern that we see in the light from stars, planets, and other objects. It looks like a thin line that is darker or lighter than the smooth colors around it. These lines appear when very small parts of matter, such as atoms and molecules, either give off or take in light.

Each kind of atom or molecule makes its own special pattern of lines. Scientists can use these patterns like fingerprints. By looking at the lines in the light from stars and planets and comparing them to lines they already know, they can find out what those faraway objects are made of. This helps us learn about what the universe is made of in ways that would not be possible otherwise.

Spectral lines are very important in astronomy and chemistry. They help us understand the tiny building blocks of everything around us, from small particles to huge stars. By studying these lines, scientists can discover new things about the materials that make up our world and beyond.

Types of line spectra

Spectral lines happen when very small parts of matter, like atoms or molecules, meet light. When a piece of matter takes in light of just the right energy, it can store that energy and then send it out again. This makes a special mark in the light pattern we call a spectral line.

We can see these lines in two ways: as dark lines where light is missing (absorption lines) or as bright lines where extra light appears (emission lines). These lines help scientists learn what substances are present, even in faraway stars, because each type of atom leaves its own unique mark.

Nomenclature

Strong lines in the light we see often have special names. For example, a line from a calcium atom might be called K.

Sometimes, lines are named based on how many extra charges the atom has. A neutral atom is called I, an atom with one extra charge is called II, and so on. So, Cu II means a copper atom with one extra charge, and Fe III means an iron atom with two extra charges. These names can also include the line’s color or wavelength. Hydrogen lines, for example, have names like the Lyman series or Balmer series.

Line broadening and shift

See also: Spectral broadening

Spectral lines are not just single points of color. They have a small range of frequencies. This happens because of different effects that change how wide or shifted these lines appear. These effects can be grouped into two types: ones that happen in a small area around the atom and ones that happen over a larger area.

Broadening due to local effects

Natural broadening happens because excited atoms have a limited time before they release energy. This short time means the energy is not exact, making the line wider.

Thermal Doppler broadening occurs because atoms in a hot gas move at different speeds. This movement changes the color of the light they emit, making the line wider as the gas gets hotter.

Pressure broadening happens when nearby atoms bump into the emitting atom. These bumps can change the energy and color of the light, making the line wider. This can happen in two ways: quickly during a collision or slowly when atoms are close for a longer time.

Broadening due to non-local effects

Opacity broadening happens when light gets absorbed as it travels through space. This can make the center of the line weaker than the edges.

Macroscopic Doppler broadening happens when different parts of a large object, like a star, move at different speeds. This also makes the line wider.

Combined effects

These effects can work together. For example, when thermal Doppler broadening and pressure broadening happen at the same time, they create a special pattern called a Voigt profile. Sometimes, these effects can even make a line narrower under certain conditions.

Spectral lines of chemical elements

See also: Hydrogen spectral series

Spectral lines are special marks that we can see in light. They are usually seen in the visible band of light, which is the light our eyes can see. But there are also spectral lines at wavelengths we cannot see.

At shorter wavelengths, which have more energy, there are ultraviolet spectral lines, like the Lyman series of hydrogen. At even shorter wavelengths, called X-rays, there are lines known as characteristic X-rays that are the same for each chemical element.

At longer wavelengths, with less energy, there are infrared spectral lines, including the Paschen series of hydrogen. And in the radio spectrum, there is the 21-cm line that helps us find neutral hydrogen all across the cosmos.

For each element, the next table shows the spectral lines that appear in the visible spectrum between 380 and 780 nanometers.