In practice, it is extremely challenging to produce a light source that emits only a single, specific wavelength of light. However, there are certain sources that can approximate a single wavelength with a very narrow range of wavelengths, making them suitable for many applications.
An example of such a source is a laser. Lasers emit coherent light, which means the light waves are in phase and have a well-defined wavelength. While lasers are not perfect in producing exactly one wavelength, they can come very close. The spectral width of a laser beam is determined by the laser's gain medium and other factors, and it can typically be quite narrow, often in the range of a few picometers (trillionths of a meter) or less.
Other sources, such as certain gas discharge lamps or atomic emission sources, can also produce light with relatively narrow emission lines corresponding to specific wavelengths associated with atomic transitions. These sources are used in spectroscopy and scientific research where precise wavelength control is required.
It's important to note that natural light sources, such as the Sun or incandescent lamps, emit a wide range of wavelengths spanning the visible spectrum and beyond. The light from these sources consists of a continuous spectrum rather than being confined to a single wavelength.
While achieving an absolute single-wavelength source is technically difficult, the use of narrowband light sources like lasers has revolutionized various fields, including telecommunications, scientific research, and numerous industrial applications.