< Physics 123, Lecture 25 Warm-Up

Warm-Up Exercise 25

Due 12:15 pm, Wed 24 Oct 2012

Physics 123, Fall 2012

Reading assignment: 35.1-35.5

In section 35.1, your book says: "The question "Is light a wave or a particle" is inappropriate, however. Sometimes light acts like a wave, and other times it acts like a particle." Do your best to explain to a friend in junior high school what that means and how it can be possible.

As Terry Pratchett would say, "It's all quantum." :-) (If you don't know Terry Pratchett yet, I highly recommend his Discworld books.) That is, quantum mechanics says that there are no such things are "particles" the way we often think of them. All real particles exhibit both "particle" as well as "wave" behavior. Macroscopic waves and particles are manifestations of the underlying quantum effects, not the other way around.

A beam of light passes through a hole of diameter d in a metal plate. Under what condition are we allowed to ignore the diffraction or “spreading” of the light?

When the wavelength is much smaller than d. This is called the "ray approximation".

In the movies or on TV, you sometimes see an actor looking in a mirror and you can see a front view of his/her face in the mirror. During the filming of such a scene, what does the actor see in the mirror?

The actor sees the camera.

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