Fraunhofer Lines

IMG_5796During this session, we make masks using diffraction grating film over the eyes to see the different colors of gases in lights. We do a rainbow flame experiment to see colors of elements as visible through light. We learn about “elements” and how elements give light color. Elements are the building blocks for all the rest of the matter in the world. Examples of elements include iron, oxygen, hydrogen, gold, and helium. Elements come in different forms; depending on their temperature they can be solid, liquid, or gas. There are currently 118 known elements. Of these, only 94 are thought to naturally exist on Earth. Hydrogen is the most common element found in the universe.

The sun's spectrum

The sun’s spectrum

Close examination of the visible-light spectrum from our Sun and other stars reveals a pattern of dark lines—called absorption lines or Fraunhofer lines. Fraunhofer lines are a set of spectral lines named after the German physicist Joseph Van Franuhofer (1787–1826) . The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun. Scientists build special instruments to separate light, usually with gratings. These instruments are called spectrographs. When astronomers pass the light of a star through a spectrograph, they get a spectrum of the star. The spectrum looks like a regular rainbow of colors—except that there are dark lines in it. It turns out that each element absorbs light of a particular frequency—a particular color. If that element is in the cool atmosphere of the star, those atoms will absorb the light at that color and produce the line. Each element has a specific “signature”—a specific set of line. These lines are the atomic markers in light – the chemical signatures in the spectrum – the colors of spectral analysis. These patterns can provide important scientific clues that reveal hidden properties of objects throughout the universe. Certain elements in the Sun’s atmosphere absorb certain colors of light. These patterns of lines within spectra act like fingerprints for atoms and molecules. Looking at the Sun’s spectrum, for example, the fingerprints for elements are clear to those knowledgeable about those patterns. We learn how images of a star’s spectra as captured by the Hubble Space Telescope give scientists clues to the age of a star and what elements it’s composed of. This forms a bridge to exploring chemistry.

Spectograph of Eta Carinae. Photo: NASA

Spectograph of Eta Carinae. Photo: NASA

When the Hubble Space Telescope views a star, it breaks up its light into different colors, like a rainbow. This is known as a star’s spectra, and if you look carefully, you can see black lines, or gaps, which correspond to certain elements. Since they can measure the ratios of different elements, astronomers can just look at a star to see how old it is. They can measure the ratio of uranium-238 to lead-206, and know how long that star has been around. Link: 4 minute NASA video featuring Eta Carinae’s spectra as seen from the Hubble Space Telescope.


Diffraction Grating Mask

Hubble Space Telescopes (Lesson coming soon)