Is the laser perfect monochromatic light source?
Nothing is perfect, but lasers are the best approximation we have - their line width can be below 1 Hz for visible light.
A perfect monochromatic light source could not be switched on/off ;).
Typical commercial lasers have linewidths ranging from kHz-GHz (He-Ne and Nd:YAG) up to a nm (laser diodes). In the lab setting, laser linewidths can go as low as 1Hz, but AFAIK that's approaching a fundamental limit.
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