Physics 23 Laboratory
Thin Film Interference
In this laboratory, we will investigate the phenomenon of thin film interference that occurs when two
waves interfere as a result of their being reflected from the two surfaces of a thin film.
I.
Reflection and transmission at a boundary.
The “before” sketches below show a wave pulse traveling from left to right and approaching the end
of the string (boundary). In the first case, the end of the string is free to oscillate, and in the second,
the end is fixed. Draw the “after” sketches for the wave pulse after it reflects from the right end of
the string.
Before:
Before:
free end
fixed end
After:
After:
The fixed and free ends of the above string represent the extremes in the way the end of the string
can be terminated. A more general boundary condition is when the right end of the string is
connected to a second string of either lower or higher linear density.
The sketches below show a pulse traveling toward a boundary created by connecting the string to
either a lighter (lower linear density) string and a heavier (greater linear density) string. Draw the
shape of the string that results after the pulse hits the boundary. Be sure to show any changes that
may occur in the width of the pulse after reflecting from and transmitting through the boundary.
Explain why the string has the shapes and widths you drew.
String connected to a string of lower linear density:
Before:
After:
String connected to a string of higher linear density:
Before:
II.
After:
Thin-film Interference
You have observed that when a beam of light strikes the surface of water, it is partially reflected and
partially transmitted. This is similar to a pulse on a string when it reaches a boundary between two
strings of different linear densities.
A.
Consider a beam of light in air (n = 1.00) incident on a soap film (just water, n = 1.33). Does the
soap film correspond to the string with the smaller linear density or the larger linear density?
Explain your reasoning.
B.
Consider light incident on a
thin soap film in a vertical
plane. A cross-sectional view
of a small portion of the thin
film is shown to the right.
air
air
soap
film
first
boundary
second
boundary
In answering the following questions, use the analogy between this situation and the connected
strings.
1.
Reflection and transmission at the first boundary.
a.
On the diagram, draw rays that correspond to the light that is transmitted and
reflected at the first boundary.
b.
Is the frequency of the transmitted wave (in the thin film) greater than, less than or equal
to the frequency of the incident wave (in the air)?
c.
Is the wavelength of the transmitted wave greater than, less than, or equal to the
wavelength of the incident wave?
d.
For light incident on the first boundary, would the reflection at this boundary be more
like reflection from a fixed end or from a free end of a string? Explain.
e.
On the basis of your answers above:
At the first boundary, would the reflected wave be in phase or 180o out of phase with
the incident wave?
At the first boundary, would the transmitted wave be in phase or 180o out of phase with
the incident wave?
2.
Reflection at the second boundary.
a.
Continue the transmitted ray (from part 1) through the first boundary of the film to
the second boundary. Then draw rays that correspond to the light that is transmitted
and reflected at the second boundary.
b.
For light incident on the second boundary, would the reflection at this boundary be
more like reflection from a fixed end or from a free end of a string? Explain.
c.
3.
At the second boundary, would the reflected wave be in phase or 180o out of phase with
the incident wave?
Transmission at the first boundary.
Continue the reflected ray from part 2 through the film back to the first boundary. Then
draw rays that correspond to the light that is transmitted and reflected at this boundary.
Would there be a phase change on transmission at this boundary?
D.
Light of frequency f = 7.5 x 1014 Hz is incident from the left side of the film.
Determine the numerical values of the:
E.

Frequency of the wave in the film

Wavelength in air (in nm)

Wavelength in the film (in nm)
Suppose that an observer were located on the left side of the film in part C.
Which of the rays that you drew could reach this observer?
thinnest part of
the soap film (n
= 1.33)
III. A film of non-uniform thickness.
The diagram shows the cross-section of a
soap film supported by a vertical loop that
has settled and is thinner at the top than at
the bottom. Light of frequency f = 7.5 x
1014 Hz is incident on the film at essentially
normal incidence.
A.
observer C
observer A
air
Observer A is looking at the part of
the film that is 75 nm thick.
air
observer B
Consider two reflected rays that reach
observer A, similar to the rays that
you identified in part E of section II.
B.
1.
How much farther does one of
these rays travel than the other in
reaching observer A?
2.
What is the phase difference
between these rays?
3.
Is observer A looking at a region of maximum brightness (constructive interference)
minimum brightness (destructive interference), or neither? Explain your reasoning.
thickest part of
the soap film
Observer B is looking at the part of the thin film that is 150 nm thick.
Is this observer looking at a region of maximum brightness, minimum brightness, or neither?
Explain your reasoning.
C.
Observer C is looking at the thinnest part of the film, where the film is extremely thin. To this
observer, would the film appear bright or dark? Explain your reasoning.
D.
Describe the appearance of the film as a whole.
E.
What are the three smallest film thicknesses for which there would be maximum constructive
interference?
F.
What are the three smallest film thicknesses for which there would be destructive interference?
G.
The thickness of the film is 1650 nm at the bottom of the film, where the film is the thickest.
Would this part of the film appear to be bright, dark, or in between? Explain.
H.
Suppose the frequency of the incident light were increased.
How would the appearance of the thinnest part of the film change?
Would the number of right and dark regions increase, decrease, or stay the same? Explain your
reasoning.