I'm doing work with a spectrometer and Hydrogen and Helium gas discharge tubes. Right now I'm studying the Balmer series for Hydrogen emission. Anyway, I'm trying to see what the "real" definition of monochromatic light is. Some references state it's light of one frequency while other references state that it's light of one wavelength? Which is it? If I look at a Hydrogen gas discharge tub, it looks "monochromatic" but when viewed through a spectrometer / diffraction grating, spectral lines at multiple wavelengths can be seen. How is monochromatic light made? Any examples of it in the physical world aside from in a lab?