When the electrons change orbits, energy is released in the form of light. Excellent, my boy. I say vacuum as they require machines called vacuum pumps to suck all the air out of their spectrometer. As I mentioned earlier, atoms can give off energy in the form of light but the reverse process is also possible, whereby the atom absorbs light. As VUV light is absorbed by atoms in the air, all the air in their instrument must be removed.
All light is part of what s called the electromagnetic spectrum.
Each different type of light has a number associated with it, called its wavelength. The type or wavelengths of light absorbed or emitted by any element constitutes that element s spectrum and no two elements are the same. By examining the light any object gives off, they can tell what it is made of simply by using these elemental fingerprints. So, you can imagine how excited that chap gets when his calcium spectrum is displayed on the screen before him. Ah, that s where things become somewhat more involved.