April 16, 2009
PHY 2054 Discussion Spring ‘09
Conceptual Review: Chapter 24
A . Conditions for Interference
1.
Two important quantities to analyze interference:
2.
Describe the condition for the phase change upon reflection.
3.
In Young’s double slit experiment, by what factor does the distance between adjacent bright and dark
fringes change if the distance between the screen and slits is tripled?
C. Interference in Thin Films
1. A beam of light w/ a wavelength of 600 nm passes through two closely spaced glass plates. What is the
minimum plate separation for the reflected beam/the transmitted beam to be bright?
2. A pair of glass plates are separated 3μm spacer along one end. The top
plate is illuminated by a light beam of wavelength 500 nm. How many
dark bands are observed on the top plate?
C. Diffraction
1. A blue light and a red light are incident on a single slit. Is the central maximum wider for the blue light
or red light?
2. A wavelength of 500 nm is sent through a diffraction grating. At what angle does the 2nd order
maximum appear if the first order maximum appears at 30º.
D. Polarization
1. Unpolarized light passes through two polarizers. The transmission axis of the 2nd polarizer makes 90º to
that of the 1st. What is the ratio of the final intensity to the initial intensity?
2. Refer to the previous question. The 3rd polarizer is inserted between the two polarizers. The
transmission axis of the 3rd polarizer makes 45º w/ respect to the 1st/3rd polarizer. Now what is the ratio?
3. At what angle above horizon is the Sun if light reflect from water surface is completely polarized?
April 16, 2009
PHY 2054 Discussion – Spring ‘09
Practice Exam Problems (Chapter 24)
1. A Young’s double-slit apparatus is set up where a screen is positioned 0.80 m from the double slits. If
the distance between alternating bright fringes is 0.95 cm and the light source has a wavelength of 580 nm,
what is the separation of the double slits? (1 nm = 10 9 m)
(Young’s Experiment)
a. 2.8  105 m b. 4.9  105 m c. 5.6  105 m d. 6.0  105 m
2. A silicon monoxide (n = 1.45) film of 100 nm thickness is used to coat a glass camera lens (n = 1.56).
What wavelength of light in the visible region (390 to 710 nm) will be most efficiently transmitted by this
system? (1 nm = 109 m) (Thin Films)
a. 400 nm b. 492 nm
c. 624 nm d. 580 nm
3. Two flat glass plates are in contact along one end and are separated by a sheet of tissue paper at the
other end. A monochromatic source of wavelength 490 nm illuminates the top plate. If 21 dark bands are
counted across the top plate, what is the paper thickness? (1 nm = 109 m) (Wedge-Shaped Films)
a. 2.7
 106 m b. 3.4  106 m c. 4.9  106 m d. 5.8  106 m
4. Light of wavelength 540 nm is incident on a slit of width 0.150 mm and a diffraction pattern is
produced on a screen that is 2.00 m from the slit. What is the width of the central bright fringe? (1 nm =
109 m) (Single-Slit Diffraction)
a. 0.720 cm
b. 1.44 cm
c. 1.76 cm d. 2.16 cm
5. At what angle will the highest order maximum appear for a wavelength 450 nm using a grating with
600 lines per mm? (Diffraction Grating)
a. 36º
b. 54º
c. 81º
d. 90º
6. Unpolarized light is passed through polarizer 1. The light then goes though polarizer 2 with its plane of
polarization at 45.0º to that of polarizer 1. What fraction of the intensity of the original light gets though
the second polarizer? (Malus’s Law) a. 0.707 b. 0.500 c. 0.250 d. 0.125
7. Sunlight reflected from a smooth ice surface is completely polarized. Determine the angle of incidence.
(nice = 1.309) (Brewster’s Angle)
Answers: 1-b
2-d
3-c
4-b
a. 25.60
5-b
6-c
b. 47.89º
7-c
c. 52.62º
d. 56.26º