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 kgm/s;
10.) 5.30  10-24 kgm/s; 11.) 2.13  10-21 kgm/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