Emission spectra are produced by excited atoms in a gas. Gas like hydrogen can be excited by heating or passing an electric discharge through it.

• Emission spectrums consist of discrete lines on a dark background.

• The emission spectrum can be observed by passing the EM radiated from the excited gas through a prism or diffraction grating to separate the discrete wavelengths.

• Andres Angstrom had measured the wavelengths of the four visible spectral lines in the Hydrogen emission spectrum.
• Balmer found an equation that enabled him to calculate these wavelengths
• Balmer’s equation for the wavelength was as follows :
  • where n , could be changed to get the various lines
  • b was empirically found to be 364.56 nm
• Rydberg modified Balmer’s equation to the following form :
  • where Balmer’s equation would be the special case with
  • is known as the Rydberg constant.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)