Textbook Problem 1B 9

[Amy Jordan 2A]

Hi, I would love some clarification on Textbook Problem 1B 9. It's "A lamp rated at 32 W (1Js-1) emits violet light of wavelength 420 nm. How many photons of violet light can the lamp generate in 2.0 s? How many moles of photons are emitted in that time interval?
I know what the solution is, but I was wondering why for the first part when you determine the number of photons you use E=hc/lambda and not use the 32 watts number because isn't the 32 watts the energy already? (I know that first you convert 32 watts to joules per second and multiply by 2 for 2 seconds). I hope this makes sense. Thank you!

[Coline Luo 2E]

Hi! I think that the 32 watts is the total power, but in order to solve this question, you need to first calculate the energy a photon in this wavelength has using the equation E=hc/wavelength. After you calculated the individual energies of the photons, you can use the given total power (converted to energy) to find how many photons are generated. Hope this helps!

[Alex_Lee_1K]

Solving for Ephotons allows you to find the minimum required amount of photons it takes to eject and electron.

Ephoton = 4.73*10^-19
From that you convert 32W * 2.00s into J so it would be = 64J
64J = # photon + 4.73x10^-19J

and then solve for # photons

Hope this helped.

[Emily Ngo 1E]

Hi Amy,

You would need to solve for the energy because you would use the equation: Etotal = #photons x Ephoton. Therefore, we need the values of Etotal and Ephoton to solve for the number of photons. Let me know if you need anymore clarification.