Photoelectric Effect – Maximum Wavelength

Hi, I am currently revising photoelectric effect, and i have this question:

A metal surface at zero potential emits electrons from its surface if light of wavelength of 450 nm is directed at it but not if light of 650nm is used. Explain why photoelectric emission happens with light of wavelength 450nm but not with light of wavelength 650nm

All i know is that the wavelength must be below a certain maximum value equal to the speed of light divided by the threshold frequency.

I dont know why this is, can someone explain why? Thanks.

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