1B.9 Question

[Stella Nguyen 1J]

Hi everyone!

I was a little confused about this problem, and I was wondering if you guys could help me with it?

Question 1B.9: A lamp rated at 32 W (1 W = 1 J ⋅ s^−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?

Thank you!

[Lucy Wang 2J]

I solved it by using the equation E=(hc)/wavelength to get 4.73 x 10^-19 Joules/photon. Then in order to get the # of photons, I divided 64J by 4.73 x 10^-19 J/photon to get the number of individual protons (1.35 x 10^20 photons). Then to convert the photons to moles use Avogadro's number to get 2.25 x 10^-4 mol.

[Bronson Mathos 1H]

For this question, you first need to determine the total energy per photon of the lamp using the equation E=hv(E=hc/lambda). Using this equation you should get: E=(6.626x10^-34J*s)(2.998x10^8m/s)/(420x10^-9m)=4.7x10^-19J*photon^-1. Using the given information that the lamp is rated at 32W(J*s^-1) we can deduce that it will emit 64J in 2 seconds, and we can divide this by our energy per photon to calculate how many photons of light the lamp can generate: 64J/4.7x10^-19= 1.4x10^20 photons. Then we can just divide this number by Avogadro's number in order to find out how many moles of photons are emitted in this time interval:(1.4x10^20photons)(1mol photons/6.022x10^23 photons)= 2.3x10^-4 mol of photons. I hope this helps!