Question

how many photons are emitted per sec by He-Ne laser that emits 1.0mW of power at a wavelength=632.8nm?how i want to relate power and the formula of energy of photons,as we know that E=hc/wavelength.

Answer 1

Well, power by definition tells you how much energy per second is used, emitted, dissipated what ever.
1 watt = 1 Joule/second

Since the energy of 1 photon is E=hc/lambda, you can calculate how much of these "packets" of energy there are emitted from the laser per second.

I hope this doesn't confuse you, I could explain better if needed.

Answer 2

First, calculate the energy of a single photon.
Second, calculate what is the total energy emitted per second from the laser (you already have that, that is power)

Now you can figure out how many photons are emitted per second, because if total energy emitted per second is X then the number of photons emitted per second is X divided by the energy of 1 photon.