What happens to a low-frequency photon when it hits a metal plate if its energy is less than the work function w0?

There is no emission of electron

Explanation:

As we know by Einstein equation

now if the initial frequency of photon is very small or if it is less than the work function of the metal then in that case the photon energy is not sufficient to eject out an electron from metal.

So here the photon is not absorbed by the metal and reflected back by the surface. So photoelectric emission is only possible when the incident photon is having energy more than the energy of the work function.

So here we can say that there is no emission of electron from that surface