Slide 2 of 18
The development of photographic film, of the gas discharge tube, and of diffraction gratings created the new science of spectroscopy in the late 19th Century.
The figure is a rough diagram of how atomic spectra were measured. The combination of lens and diffraction grating created an image of the slit at a different place on the film for each wavelength of light. If the atom emitted a single wavelength, a strong image of the slit showed up as a dark line on the developed film, so the wavelengths emitted were called “spectral lines.” There was no color film then, but the diffraction grating equation allowed wavelengths to be calculated by measuring the lines on the film.
Each chemical element turned out to have its own “fingerprint” or set of wavelengths that it emitted when excited electrically in the tube. This obviously had something to do with the internal structure of the atom, but what? Metaphorically speaking, the spectra were telling the story of the atom, but in a language nobody understood.