Introductory Physics Volume Two

152 Wave Optics 7.9
angles for the minima and maxima.
d sin θ m = m λ { max m is even.
2 min m is odd
⊲ Problem 7.10
You set up a sheet of aluminum foil to block a microwave transmitter.
You now punch two holes in the foil, so that the microwaves pass
through the holes. The holes are 10 cm apart. You now place a microwave
detector on a track that is parallel to the foil and 3 meters away.
By sliding the detector along the track you observe that the response
of the detector is as shown in the diagram. What is the wavelength of
the microwave?
Foil with holes
x
120cm
80cm
Source
10 cm
Detector Response
as a function
of position
40cm
0cm
300 cm
§ 7.9 Thin Film Interference
The swirling colors that you see reflected in a
soap bubble and in puddles in a parking lot are a
result of interference. Let us start by considering
the colors in the puddles. The reason that you
only see this in a parking lot is because it only
happens when there is a thin film of oil on the top
of the water. Consider the diagram to the right,
Oil t
of the layers of a puddle. At the bottom we have
the pavement of the parking lot. Next there is the
pool of water. On top of the water is a thin layer of
Water
Pavement
oil. The thickness of the oil has been exaggerated
so that we can see what is happening.
The light from the sun strikes the surface of the oil and some of it
is reflected back toward the detector (your eye). The rest of the light