Why doesn't a charged metal plate lose charge under certain light?

[Marshall878]

Im trying to get my head around a question in a book as i thought it would be good to give them ago as I am struggling.

The question is:

Explain why a metal plate does not lose its charge when;

a) the plate is + charged and illuminated with visible light.

b) the plate is - charged and illuminated with UV light

Also, when given wavelength and power of a wave, how do you work out the number of photons emitted per second?

 

[Eivind]

a)Even though the plate are being exposed for visible light, the light has not enough energy to free electrons from the metal.

b)The plate has already a negative charge, and won't become positive if you expose it to UV light. The effect releases electrons, its not filling up whit them.

 

[Lucretius]

I have a question about the photoelectric effect. Does it cause metal to decay? It involves emission of electrons from the metal and as far as I know there are no electrons being replaced: does the light cause the metal to break down after a long period of time?