Electro-Magnetic Waves I PREFLIGHT

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The following three questions refer to the material you were to read in preparation for the lesson. Questions one and three require you to write a three or four sentence response. Number two is a multiple choice question. Click in the appropriate circle.

You may change your mind as often as you wish. When you are satisfied with your responses, click the SUBMIT button at the bottom of this page. Don't submit more than once. (If you absolutely HAVE to resubmit it, put a note on the end to that effect.)




1.

One of the strong contenders in advanced propulsion for 21st century spaceflight is the solar sail. Contrary to popular opinion, the solar sail does NOT use the solar wind, but the light from the sun. How is this possible? Should the sail be white or black?
(A page of more information is at: http://www.ugcs.caltech.edu/~diedrich/solarsails/. A cool page (in French) that describes how to fold a solar sail is at: http://www.ec-lille.fr/~u3p/textang/pliage1a.html Another Science News article by a classmate of mine: http://www.sciencenews.org/sn_arc99/8_21_99/bob1.htm).




2.

In the figure at the left, the electric field of an electromagnetic wave at a certain point and certain instant is given. The wave is transporting energy (Poynting vector!) in the negative x direction. What is the direction of the magnetic field at that point and instant?

Left (-x)

Right (+x)

In (-z)

Out (+z)


3.


3D Wave Physlet

In the simulation at the left, an EM wave is shown impinging on a polarizer. Even though the polarizer and the wave are not lined up, some light gets through. Estimate how much light makes it through the polarizer and give a physics explanation why you expect this much light to "leak" through.

Click-drag to the right or left to rotate about the z axis.  Click-drag up or down to rotate in the xy plane.

Requres Java 1.1 browser such as IE 4.01 or Communicator 4.05 with Java patch from Sun.






honors extra


Although Solar Sailing has been talked about for decades (Arthur C. Clarke wrote a science fiction novel on the subject at least 30 years ago) a scientist at the University of Washington, Robert Winglee, has turned the whole debate upside down when he proposed a propulsion that DID use the Solar Wind. Marshall Space Flight Center is very interested, and I may spend my summer testing out a small version of Winglee's "M2P2" in the humongous vacuum chamber at MSFC. Here's a web page of Winglee's idea: http://www.geophys.washington.edu/Space/SpaceModel/M2P2/. The thing that startled everybody was that "everyone knew" that the propulsion from light was a lot stronger than the propulsion from Solar Wind. So a patch of solar sail should get pushed a lot more by light than by solar wind. If you knew that the solar wind had a density of 3 ions/cc, and was travelling 400 km/s, while the sunlight had an intensity of 1.4 kWatt/m2, can you estimate the ratio of sunlight pressure/solar wind pressure? (And why then, is Winglee being taken seriously?)






Below is a space for your thoughts, including general comments about today's assignment (what seemed impossible, what reading didn't make sense, what we should spend class time on, what was "cool", etc.):




You may change your mind as often as you wish. When you are satisfied with your responses, click the SUBMIT button.

I received no help from anyone on this assignment.