1. A 70-W green laser (lambda = 514 nm) shines uniformly in all directions. a. (
ID: 1698784 • Letter: 1
Question
1. A 70-W green laser (lambda = 514 nm) shines uniformly in all directions.a. (2 pts) What is the energy of the photons emitted by this laser [in eV]?
b. (3 pts) How many photons does the laser emit per second?
c. (2 pts) Potassium has a work function of 2.30 eV. What is the cutoff frequency [in THz] for the photoelectric effect on potassium? Will the laser cause electrons to be ejected from a potassium surface?
2. (3 pts) What is the kinetic energy [in eV] of the electrons ejected from potassium if illuminated by a UV laser (lambda = 238 nm)?
Explanation / Answer
a)E=hf=hc/lamda=3.87e-19(J)=2.4(eV) b)70/3.87e-19=1.81e20(photon/sec) c)2.3eV=2.3*1.6e-19(J). so hf'=2.3*e so f'=5.56e14(hz) ---- yes because 2.4>2.3 d) E=hf=5.21(eV) so K=5.21-2.3=2.9(eV)
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