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[Phys-l] A geek's observations on "Avatar"

I haven't contributed anything to this group in a long time, but thanks to an improvement in my physical health I feel inspired to do so again. This won't be deep physics, but it is interesting, especially for teachers going back into classrooms tomorrow.

I, my wife, two of our sons, and a family of four friends saw the remarkable film "Avatar" in RealD 3D. A member of our AAPT section had kindly posted a technical writeup* which sons David, Aaron and I had read in advance. The experience was well worth the time and money spent, especially since Aaron treated us all!

My principal interest was in seeing the finished product. It was not quite perfect, I'm afraid, but it was much, much better than any previous 3D movies I had seen. The rendition is effectively at 24 frames per second, regular movie frame rate. As a consequence the motion is not smooth, something that showed up dramatically in the 20th Century Fox logo before the start of the film itself. As the CGI image rotates, some small objects at the bottom of the screen are supposed to appear to be moving rapidly from right to left. They don't; they jump across instead in ten or fifteen discreet steps each. Once I had recognized this "shortcoming" I continued to watch for it in the movie, but I really didn't see another instance where it was noticeably annoying. As I understand it, the problem could have been addressed by the in-betweening process of computing intermediate images up to the 72 frames per second rate at which the images are projected to each eye, in the same manner that modern TV displays produce 120 Hz and 240 Hz frame rates to smooth motion.

The other thing that really interested me is the glasses. They are circular polarizers that alternately transmit and block light reflected from the screen. I stood outside the theater after the show and collected half a dozen pair because I knew they would be fun to play with. The family's nine and eleven year old girls were fascinated by the tricks I could do with the lenses. Of course I have played with circular polarizers a lot previously, so I already knew exactly what to expect, and I was able to put on a good series of demonstrations, including using them as simple plane polarizers, showing that light reflected from a kitchen floor is strongly plane polarized, for example. I also showed how a single lens could suppress reflections from a shiny coin, and that the antireflective capability cold be destroyed by simply adding a second filter in series. It was a remarkable instance of a teachable moment, even though my only handy students were 10- and 13-year-old girls. I haven't yet removed any of the lenses from the frames, but doing so will make them easier to use in demonstrations.

It is interesting to note that the package in which the glasses were handed out says:

These glasses do not screen ultraviolet
light. Wearing them as sunglasses will not
protect your eyes. Use only in the theater.
Keep out of reach of children under 3."

This is a CYA disclaimer, of course, but it is interesting to note that the glasses can be used to function like Polaroid sunglasses - to reduce glare from water or pavement, rather than to screen out ultraviolet radiation. However this can only be done by modifying their use; they won't work as provided in the theater. For the pairs I examined the lenses work as plane polarized light blockers if one looks through them backwards. However the plane polarized light they transmit is horizontally polarized, and that's the component that must be blocked to reduce glare. Thus to use them as ordinary polarized sunglasses you must remove the lenses, reverse them front-to-back, and also rotate them 90 degrees about the line of sight. That would scarcely be worth the effort, by the way.

If you take a child to the movie a child-sized pair of glasses will be offered. Before accepting them be sure your child cannot use the full sized glasses. The whole screen cannot be viewed within the width of the child sized lens from a reasonable distance. We were sitting three rows above the middle aisle, and I sent the 13-year-old back to trade glasses before the film started. I should have sent the ten year old back too, as it turned out.

If you are a geek you must see this film, and you should read the short pdf* first. Don't forget to collect some extra glasses after the film is over. The two lenses are different, and you need at least two of each kind to explore their fascinating properties. If you don't know how the circularly polarizing lenses work and you want to know, I can give you little help. A brief Googling yielded one good result, but I could read only the cached version. You can reach it via Googling "circular polarizer 45" (without the quotes, of course) and clicking on the cached version of <> which shows up on the second page of results. Briefly, a circular polarizer consists of a linear polarizer laminated to a quarter wave retardation plate at 45 degrees to its fast axis. Scarcely a good description. (The textbooks I have don't include good descriptions either.)


*See <>. This writeup is by Matt Cowan, q.g.