Why is it difficult to obtain a strictly monochromatic light of a single wavelength?

Question

I read that it's difficult to obtain a monochromatic light of a single wavelength. We usually have a range of wavelengths associated with it. Why so?

I saw a similar question posted here: https://www.researchgate.net/post/Can_we_have_strictly_monochromatic_unpolarized_light

I couldn't understand the answers here properly and moreover the question talks of polarization of light as well. Has that got to do anything with my original question?

Answer 1

Strictly monochromatic light would require your wave train to be infinitely long. As soon as you include a "switch-on"-moment (or switch-off) the signal contains more than one frequency. To see this, just calculate the Fourier transform of such a wave.

So much about the theoretical problems with monochromatic waves. If you technically want to realize quasi-monochromatic light, you have some more problems:

• You could filter an arbitrary light source with many successive filters, but in the end you don't only lose the majority of the intensity, but also sooner or later run into the same problems as you have when frequency-stabilizing a laser. This is because narrowband filters are cavities; similar to those used for lasers.
• Lasers are bright sources for spectrally narrow light. But still they show fluctuations of their emission wavelength, because the resonace frequency of their cavity changes all the time. This is caused mainly by vibrations, but also other influences like thermal expansion or changing refractive index within the cavity.

In the Researchgate question they actually ask something different, namely whether there is a possibility to have monochromatic light that is unpolarized.