In an important experiment used to determine the nature of atoms, a gas like hydrogen, neon or mercury is excited by sparking it with electricity. The light emitted is separated out into individual colors using a prism or diffraction grating and a few lines of colored light are seen. The light observed in such a spectrum is emitted when an electron in an atom

"jumps" from one energy level down to another lower energy level.

Explanation:

The light observed in such a spectrum is emitted when an electron in an atom "jumps" from one energy level down to another lower energy level. When an electron does this, it emits energy equal to difference in the energy level of two shell in which the electron jumps.

And this emitted light is separated out into individual colors using a prism or diffraction grating and a few lines of colored light are seen.