Under what condition would particle de Broglie wavelength be equal to the classical EM wavelength?
A particle is different from an EM wave and so it doesn't have an EM wavelength. So there is no special value to call "the classical EM wavelength". And EM waves can have any wavelength, so you can find some EM wave with a wavelength equal to the de Broglie wavelength of some particle but there is no meaning behind it. Its like picking an apple and asking whether there is any car with the same color as this apple. Yes, there is. But...what's the point?
Also the de Broglie wavelength is something that is no longer used. Its an obsolete concept. It was there when people still didn't completely understand quantum mechanics.
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