Quantum Theory & Atomic Structure: Calculating number of photons?

Question

The guy who answered this yesterday was totally off.
I'm given the minimum amount of energy, 2.0x10^-17 J. I have to calculate how many photons of red light (700 nm) are needed to reach that. How do I do that?

I know how to calculate the energy of the photon. But how do I get the number of photons???

Answer 1

According to the defintion of a Photon ,you only need one photon ray at the frequency of 3.18377922 x10^16 herz.to obtain 2 x 10^-17 joules.
In the same way you only need one photon ray at a frequency of 4.2827494 x10^14 hertz to produce an energy of 2.837782087^-19 joules falling per unit area..

So the total energy falling on the same area would means you would have 74.33961044 photon rays to to produce red light on that area.You basically have a laser flux.

Answer 2

The Planck's theorem says : Energy of a photon is E = hv
where h is Planck constant which equals to 6.626x10^-34 J.s, v is the photon frequency.
Your frequency is : v = c/lambda = (299 792 458 m/s) / (700x10^-9 m) = 4.2827x10^+014 (Hz)
Energy of one photon of red light is : E = 6.626x10^-34 (J.s) x 4.2827x10^+014 (Hz) = 2.8377x10^-019 (J)
Therefore, number of photons : N = E_tot/E = 2.0x10^-17 / 2.8377x10^-019 = 70

Answer 3

I tell you a simple formula for this kind of questions.
Always in case of EM radiations E = hc/w where E is the energy , h is the planck's constant , c is the velocity of light in vaccum and w is the wavelength of radiation. Use c= 3*10^8.

Answer 4

The answer to all of them is 42.