
84 Questioning the Universe: Concepts in Physics
The answer: By the process of elimination. When observing the full spectrum
from the sun, the spectra from all the known elements could be eliminated. When this
was done, there were still a set of discrete frequencies left over. Thus, an unknown
element had to exist on the sun that had not yet been observed on earth. Helium was
nally detected on earth in 1895, when the spectrum from an unknown gas was
shown to be the same as that observed from the sun.
9.7 THE ELECTRON VOLT: A USEFUL ENERGY UNIT
Above, we have talked about the energy levels in the atom, but did not mention how
big these energies ...