Only newer instruments were thought to be needed to get more precise measurements. Sunlight is composed of visible electromagnetic radiation of wavelengths between 400 and 750 nm and other radiation as well. Scientists had argued about the nature of light for years. At the beginning of the twentieth century, the wave theory predominated. This was the result of a diffraction experiment.
Light waves add when they are in phase and subtract when out of phase.
Matter was not known to subtract. The longer the wavelength, the lower the frequency, the shorter the wavelength, the higher the frequency. The range of frequencies or wave lengths is called the electromagnetic spectrum - it ranges from gamma rays to TV, FM, AM radio waves. A continuous spectrum results from an incandescent light bulb. The gas glows.