Physics help

[Adam Preston]

hi, I'm having trouble with a question my teacher hasn't covered in my unit of work on photons, this sort of question may come in my exam after sitting my first miserable 1, anyway, if you guys could give me hints that would be most appreciating so I can work through getting the answer myself.

Question :
A light with the an intensity of 1mW/cm^2 falls upon the surface of a metal, the wavelength of the light is 633nm. How many photons will fall onto 1cm^2 of the metal every second?
I have no idea where to begin as pretty much all the question we've done is calculate the frequency, wavelength of light and energy of photons. Any help is welcomed.

[eSOANEM]

How is a watt defined in terms of energy?

This, combined with the energy per photon should give you the answer by looking at the units.

[Adam Preston]

So, a watt is the amount of joules per second, by using the wavelength I work out the frequency and then the energy of a photon.Then do I divide the energy of the photon with the amount of joules per second to get the amount of photons?

[gmalivuk]

Yes, you will do some division. To figure out which number you divide by which other number, consider the dimensions involved.

You are given power / area and an area, which you multiply to get total power, which is energy / time. You can also figure out the energy of each photon, which gives you units of energy / photon. What you want is photons / time. Which number do you divide by which to get that?

[eSOANEM]

If you're ever unsure how to solve a problem like this, in most cases a dimensional argument will give you the correct answer (provided you're in consistent units). Also, in such cases, it's important to consider the "dimension" of things you'd usually consider dimensionless (such as number or angle e.g. e=1.602*10^-19 C/electron or angular velocity = radians/second).

[Adam Preston]

ok i'll try and look at it like that