In 1814 the Austrian physicist Fraunhofer observed the Sun’s spectrum. Through a tin hole he allowed ? sheaf of light, to be directed to refractive prism. Fraunhofer noticed that the unbroken Sun spectrum has many dark lines – these are called a linear absorption spectrum (see the figure to the left).

Every chemical element in the Universe has a nucleus and an electronic cloud around it. Hydrogen, for example, has only one electron, which reacts on some strictly defined wavelengths. One red photon has the exact energy necessary for the electron to move up one energy level. The result is that in the spectrum this red photon is missing. A black line replaces it. A green photon, which has more energy, makes the electron jump two levels. The same happens with a blue photon, which sends the electron up three levels, and with the violet which sends it up four or more levels. Only the hydrogen atom can absorb simultaneously these four photons and no others. Its absorption spectrum is its signature and it can’t be falsified.