Physics 202 MWF 10:20 Spring 2004 (Ford)

Exam IV Chapts. 27, 29, 30 in Cutnell and Johnson 6e


Multiple choice questions. Circle the correct answer. No work needs to be shown and no partial credit will be given.


(4 pts) 1. A thin film of a material is floating on water (n =1.33). If the material has a refractive index of n = 1.45, does the film look bright or dark in reflected light as its thickness approaches zero?

(a) bright
(b) dark<-------- Ans.


(4 pts) 2. Which of the following will decrease the width of the central diffraction pattern produced when coherent light of wavelength and frequency f passes through a single slit?

(a) decrease the width of the slit
(b) decrease <-------- Ans.
(c) decrease f
(d) all of the above


(4 pts) 3. Which of the following will increase the maximum kinetic energy of the photoelectrons ejected from a surface when light of wavelength and frequency f shines on the surface?

(a) increase the intensity of the light
(b) increase the work function of the surface
(c) increase f<-------- Ans.
(d) increase
(e) all of the above


(4 pts) 4. When the momentum of a photon doubles, the energy of the photon

(a) stays the same
(b) doubles<-------- Ans.
(c) becomes four times as great
(d) increases by a factor of 2^1/2


(4 pts) 5. A rock is dropped from the top of a building. As the rock falls, its de Broglie wavelength

(a) increases
(b) decreases<-------- Ans.
(c) stays the same


Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(14 pts) 6.

a) What is the energy of a 500 nm photon?

Ans. ___3.98 × 10^-19 J = 2.49 eV_____

b) What is the energy of an electron that has de Broglie wavelength 500 nm?

Ans. __6.03 × 10^-6 eV = 9.65 × 10^-25 J_____


(16pts) 7.

a) Use the Bohr model to calculate the de Broglie wavelength of the electron when the atom is in the n = 5 energy state.

Ans. ____1.66 × 10^-9 m____

b) A hydrogen atom is in a state for which the Bohr model says that the angular momentum is 3(h/2). What is the energy of the atom, in eV? Take the ground state energy of the atom to be -13.6 eV.

Ans. ____-1.51 eV___


(16 pts) 8. In a Compton scattering experiment the incident photon has wavelength 0.01400 nm. The photon scatters backward ( = 180°).

a) What is the energy of the electron after the photon scatters from it?

Ans. ____3.71 × 10^-15 J = 2.32 × 10⁴ eV____

b) What is the magnitude of the momentum of the electron after the photon scatters from it?

Ans. ____8.22 × 10^-23 kg·m/s_____


(18 pts) 9. a) Coherent light of wavelength passes through a single slit of width 0.60 mm. On a screen 5.0 m from the slit the central maximum of the diffraction has a width of 4.0 mm. Calculate .

Ans. ____240 nm___

b) Coherent light of wavelength 500 nm passes through a diffraction grating. On a screen 5.0 m from the grating the distance between adjacent bright fringes near the center of the screen is 4.0 mm. What is the spacing between adjacent slits of the grating?

Ans. ____0.625 mm____


(16 pts) 10. Each side of a square has length 4.0 m. Two identical in-phase sources of electromagnetic waves at corners P and Q emit identical waves. At point A at the midpoint of the opposite side of the square the waves interfere constructively. Along this side of the square the next point where there is constructive interference is point B, at one corner. What is the wavelength of the waves?



Ans. ____1.65 m____