Why is the atomic emission spectrum of hydrogen composed of lines? What does each line indicate? Why do the lines become closer together from left to right in a typical diagram?

Answer 1

The hydrogen atom with its one electron in the N=1 shell is in it's "ground" state. When it absorbs a photon of energy of just the right amount, the electron is promoted to a higher energy level such as N = 2, N = 3, N = 4, etc. This "excited" atom stays in that state a nanosecond or so before the electron falls to a lower energy level. When that happens the atom emits a photon of energy equal to the difference between the starting and ending energy levels. Each transition from a higher level to a lower level results in a "line" of a particular wavelength when viewed with a device that separates the wavelengths such as a spectroscope, spectrograph or spectrophotometer. The lines are closer together toward shorter wavelengths because the energy levels are closer together as they progress from N = 1 to N = infinity.