This means that it doesn’t reflect light (and so cannot act as a mirror) and it doesn’t let light pass through it. A blackbody absorbs all of the light that hits its surface. Kirchhoff coined the term ‘blackbody’ to describe a hypothetical object that emits a continuous spectrum, with no absorption or emission lines. In the 20th century, Niels Bohr’s theory of the atom was used to explain why particular elements are associated with particular energies (discussed in Chapter 10).īlackbodies, like stars, produce a continuous spectrum, absorption lines are caused by hot gases that absorb some of this light, and emission lines are caused by hot gases emitting light. In 1885, the Swiss mathematician Johann Balmer discovered an equation linking the energies of all the hydrogen lines in the visible spectrum and the Swedish physicist Johannes Rydberg improved upon this equation in 1888. In 1864, the British astronomer William Huggins and the Irish-British astronomer Margaret Huggins showed that stars are made of some of these elements and that they are mostly made of hydrogen. Kirchhoff and Bunsen determined the energies of lines produced by different elements in the laboratory. These clouds can then emit this light at the same specific energies, creating emission lines. ![]() These are caused by clouds of gas that absorb some of the star’s light before it reaches Earth. The dark lines found in the spectra of stars are absorption lines. In the 1860s, the German natural philosophers Gustav Kirchhoff and Robert Bunsen showed that spectral lines are caused by different chemical elements absorbing or emitting light at specific energies. ![]() v is the velocity at which it’s moving, c is the speed of light, and ν is the frequency.Įdwin Hubble later discovered evidence for the big bang when he showed that almost all galaxies have redshifted spectra and are therefore moving away from us (discussed in Book I). Here, λ is the wavelength where the spectral line appears for light from the Sun and Δ λ is the change in wavelength between this and the wavelength where the spectral line appears for light from something moving relative to the Sun. Reflection, Refraction, and DiffractionĢ0.
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