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Physical Optics

Q: If you put a few drops of liquid in a cup of soap with a little water and shake it around, making a great froth of bubbles. After several seconds, the bubbles will begin to show bright rainbow color patterns. Why must you wait before the colors appear?

Q: If you take a very close look at a worn surface under direct sunlight. A fingernail, an old coin, or the roof of an old car will all show a very fine granular pattern of tiny colored dots. This phenomenon is the so-called "speckled effect". What might be it's cause? Notice how it seems to move as you slowly move your head parallel to the surface.

Multiple choice:
1. Natural light is light that is: (a) only found in nature (b) linearly polarized (c) completely unpolarized (d) polarized but rapidly changing (e) none of these. Explain your choice.

2. Unlike transverse waves, longitudinal waves cannot: (a) interfere, (b) diffract (c) be reflected (d) be polarized (e) none of the above. Explain your choice.

Problem:
1. A film of oil (nf = 1.42) 0.000 02 mm thick is spread ona glass plate and viewd from above at nearly 90 deg to the surface. What color will the film appear to be? Compute the vacuum wavelength.

2. A glass camera lens with an index of 1.55 is to be coated with cryolite film (n = 1.30) to decrease the reflection of normally incident green light (lander 0 = 500 nm). What minimum thickness should be deposited on the lens?

3. A narrow vertical single slit (in air) is illuminated by IR from a He-Ne laser at 1152.2 nm, and it is found that the center of the first dard band lies at an angle of 4.2 deg off the central axis. Determine the width of the slit.

Solution Preview

Q: If you put a few drops of liquid in a cup of soap with a little water and shake it around, making a great froth of bubbles. After several seconds, the bubbles will begin to show bright rainbow color patterns. Why must you wait before the colors appear?
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<br>A: The liquid film must become thin enough, 1/2 or 1/4 wavelength of visible light.
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<br>Q: If you take a very close look at a worn surface under direct sunlight. A fingernail, an old coin, or the roof of an old car will all show a very fine granular pattern of tiny colored dots. This phenomenon is the so-called "speckled effect". What might be it's cause? Notice how it seems to move as you slowly move your head parallel to the ...

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