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Modern Physics – Practice Problems Note: 1 eV = 1.602 10-19 J 1. A helium-neon laser produces light with a frequency of 4.74 1014 Hz. What is the energy of the photons produced by the laser? 2. What is the energy of a photon that has a wavelength of 600 nm? 3. How many photons are emitted from a 1.5 10-3 W laser each second if the frequency of the laser light is 4.75 1014 Hz? 4. A photoelectric surface has a work function of 3.30 10-19 J. What is the threshold frequency of incident light that will cause electrons to be emitted from the surface? 5. Light with a wavelength of 530 nm falls on a photoelectric surface that has a work function of 1.70 eV. What is the maximum energy of the ejected electrons? 6. Light with a wavelength of 425 nm falls on a photoelectric surface that has a work function of 2.0 eV. What is the maximum speed of the ejected electrons? 7. Electrons are emitted from a photoelectric surface with a maximum speed of 4.20 105 m/s. If the work function of the surface is 2.55 eV, what is the wavelength of the incident light? 8. What is the “stopping” voltage of an electron that has 5.40 10-19 J of kinetic energy? 9. Calculate the momentum of a photon whose frequency is 9.65 1014 Hz. 10. Calculate the momentum of a photon whose wavelength is 1.25 10-10 m. 11. Calculate the momentum of a 4.00 MeV photon. 12. Calculate the wavelength of an electron that has a speed of 2.25 107m/s. 13. Calculate the wavelength of an electron that has a kinetic energy of 7.50 10-3 MeV. 14. What is the frequency of an electron that has a speed of 9.20 105 m/s? 15. Calculate the energy of the 3rd energy level of the hydrogen atom. 16. If an electron in a hydrogen atom has an energy of –0.544 eV, what energy level must it be in? Answers: 1.) 3.14 10-19 J; 2.) 3.32 10-19 J; 3.) 4.76 1015 photons; 4.) 4.98 1014 Hz; 5.) 1.03 10-19 J; 6.) 5.7 105 m/s; 7.) 407 nm; 8.) 3.38 V; 9.) 2.13 10-27 kgm/s; 10.) 5.30 10-24 kgm/s; 11.) 2.13 10-21 kgm/s; 12.) 3.23 10-11 m; 13.) 1.42 10-11 m; 14.) 1.16 1015 Hz; 15.) –1.51 eV; 16.) 5th energy level