system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we have a perfectly monochromatic light?

In my textbook, it says that (in the context of the double slit experiment) that a perfectly monochromatic light will be ideal for the experiment, but that simply isn't possible to obtain, and a range of about 10-15 nm wavelength is used. Why can't we have an exact wavelength? Is it because of the imperfections in the way we produce the light or is it something more fundamental like the Uncertainty Principle?
gpt
Even if you had some hypothetical perfectly monochromatic emitter, the process of turning it on and off introduces other wavelengths into the equation: the Fourier transform of a wave packet includes many terms. The only way you could have purely monochromatic light would be to have a monochromatic wave of infinite extent.

However, it's not really possible to have a perfectly monochromatic source in the first place. Transitions between atomic energy levels (the main mechanism behind lasers) are the best sources of monochromatic light, but even these have finite spectral linewidths due to thermal motion of atoms (Doppler broadening) and interatomic or electromagnetic interactions (Stark, Zeeman effects).