What Are Fraunhofer Lines?

Photosphere, visible surface of the Sun from which most of the sunlight that reaches Earth directly is emitted. Because the Sun is so far away, the edge of the photosphere appears sharp to the naked eye, but in reality, the Sun has no surface, being too hot for matter to exist in anything other than a plasma state – that is, as a gas from ionized atoms.

Scientists consider the Sun’s “surface” to be the region over which most photons (the quantum carriers of light energy) escape. So, the photosphere is a layer about 400 km (250 miles) thick. Temperatures in this layer range from 4,400 Kelvin (K; 4,100 °C or 7,400 °F) at the top to 10,000 K (9,700 °C or 17,500 °F) at the bottom.

Photons generated deeper cannot get out without absorption and re-emission. The density of the ionized gas is about 1/1,000 that of air at Earth’s surface, but is much more opaque due to the strong absorption of light by the hydrogen ions.

What Are Fraunhofer Lines?

Fraunhofer lines, in astronomical spectroscopy, are any dark (absorption) lines in the spectrum of the Sun or any other star, caused by selective absorption of the Sun’s or star’s radiation at certain wavelengths by the various elements known as gases in its atmosphere are present.

The lines were first observed by the English physicist William Hyde Wollaston in 1802, but are named after the German physicist Joseph von Fraunhofer, who recorded more than 500 of them from about 1814 and labelled the brightest ones with the letters A through G, an identification system yet in action.

It is now known that about 25,000 Fraunhofer lines exist in the solar spectrum between the wavelengths of 2,950 and 10,000 angstroms. (One angstrom equals 10-8 cm.)

Why are Fraunhofer lines important?

When these lines were first discovered, they were simply an anomaly that was not understood. Today we understand that Fraunhofer lines are caused by the elements in the gas absorbing some of the wavelengths of light.

For example, if we shine light through hydrogen gas, then the wavelengths at 365 nm, 656 nm, and 820 nm are some of the most important visible Fraunhofer lines because these are the wavelengths of light that are absorbed by hydrogen.

This information can be used for many applications. One of the most widespread uses is to determine what gases make up the Sun and other stars. By observing which Fraunhofer lines can be seen in sunlight, we can determine what gases make up the sun. Fraunhofer lines were also used to determine the structure of atoms.

How Fraunhofer lines are created?

Fraunhofer lines come into play when the continuous spectrum is affected by the gas. When the white light passes through a gas, these atoms absorb part of the wavelengths. Can you guess what wavelengths are being absorbed? Each gas atom absorbs the specific wavelength or colour that it emits.

So, when the white light finally reaches its destination, the spectrum is not complete. It will lack these absorbed wavelengths. If you look at the continuous spectrum, there are thick black lines everywhere. These are called Fraunhofer lines.

By noting which wavelengths were absorbed, scientists can determine which elements are present in the sun. In fact, the Fraunhofer lines led to the discovery of helium. In 1870, helium was an unknown element. All scientists at that time knew that this element occurs in the sun. It wasn’t until nearly 25 years later that the same element was discovered here on Earth.

To date, there are over 25,000 Fraunhofer lines in the colour spectrum of our solar system. The lines are still used today to study distant stars and other celestial bodies.

References:

  1. Fraunhofer Lines: Definition & Table. https://study.com/academy/lesson/fraunhofer-lines-definition-table.html
  2. What Are Fraunhofer Lines? https://theplanets.org/what-are-fraunhofer-lines/