Why in the experiment to observe the photoelectric effect, it is necessary to charge the zinc plate negatively?

Because light rips out electrons. Under the influence of light, the energy of electrons in atoms increases, and they can leave the plate. With a negative charge, the torn electrons are repelled from the plate and the electrometer is discharged. With a positive charge, the field returns the torn electrons back to the plate and the electrometer does not discharge.

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