You enter the classroom on the first day of your first course in college physics.  The windowless room is dimly lit by several lamps on the demonstration table in the front of the room.  When everyone is seated, the instructor shuts the doors, and raises the large projection screen to reveal several large paperback books standing upright in the chalk tray. 
    The instructor announces he is about to show us something about vision that may surprise some of us.  As he speaks he briefly illuminates one of the books with a small pocket flashlight.  The book is illuminated by a spot of bright green light.  He says something about light being "photons" that carry forces of some kind and shines the light on a book again.  This time the light is bright yellow.  The next time it's red.  Next purple.  Then orange.  Very strange...
    The flashlight goes back in the pocket and he asks if anyone has any questions so far.  "Yes," you think.  Somebody else asks it:  "How did you change the filter on the flashlight so fast?"  He answers by taking out the flashlight again, flashing it quickly on each of the books.  Purple.  Blue.  Green.  Yellow.  Orange.  Red.
    "Like this," he says.  Then he turns on the room lights, and it's obvious that there weren't any filters.  The books all have brightly colored covers.  "You were all totally colorblind after you entered the room," he says as he shines the flashlight in our direction.  The light is white.  "Did you notice?" 
      "These lights are low-pressure sodium vapor lamps," he explains.  "Their light is just a very narrow band of wavelengths and that was the only light you were seeing by.  Color distinctions are impossible.  Your eye and mind work together though, and try to compensate for that kind of lighting problem.  You probably saw white as something like white and just didn't notice that colors simply weren't there.  Everything was really just shades of yellow."
    Then he holds up a yellow glass filter and puts it  over a small fluorescent lamp.  Its color is virtually identical to the color of the sodium lamps, one of which is now facing the class.  Then he holds the yellow filter over the lamp, and absolutely no light gets through. 
    Is that right?  The yellow filter blocks out the yellow sodium light? He asks the class to jot down first impressions of what just happened.  The class is to be ready to compare notes.
    "Color vision is a bit more subtle than it might seem," he continues.  "About 5% or so are colorblind in one of several possible ways.  Most colorblind individuals are men.  Many colorblind people don't know they are colorblind." 
    He looks over the class and asks, "Who is pretty sure he is not colorblind and would like to see how well he discriminates different colors?"  Somebody volunteers and is seated, facing the class, on a high stool at the front of the room.  "Now fix your gaze down the center aisle, and we will test your peripheral color vision."
    The instructor brings the bright red book slowly from the rear toward the side of the person on the stool, shaking it up and down a little bit.  The subject says, "There it is!" and the instructor quickly returns it back and out of sight.
    "What color was it?"
    A bit of hesitation.  Then a tentative, "Green?"  The instructor takes the green book and repeats the experiment.
    "Ha," says the subject.  Back to the rear goes the book.  But the color name doesn't come easily. . . "Blue?"
    "Are you sure you're not colorblind?"
    "I don't think so."
      The class  buzzes a little, and after a couple more attempts to identify the colors, somebody else asks to try.  But the result is the same.  The second person seems colorblind, too.  But this time the instructor shows the subject the book after each try.
    "Any suggestions?" says the instructor.
    Somebody says, "Peripheral vision is colorblind."
    "Right."
    The instructor distributes a small manila envelope to each person in the class asking everyone to handle its contents carefully and not put fingerprints on what look a lot like a bunch of 35mm slides.
    The "slides" are labeled, "diffraction grating," "polarizer" (2), "846," and "866."  The last two are dark purple filters, one darker than the other.  The polarizers have heavy black lines labeled "plane of polarization."
    "Look at things in the room through those devices," he says.  "The sodium lamp, the fluorescent lamp, the incandescent ceiling lamps.  The fluorescent ceiling lamps.  Look at the reflections off the shiny surfaces in the room.  Look at the light from the yellow filter."  (It was in a holder in front of the little fluorescent lamp.)  "Discuss what you see with your neighbor."
    Soon the classroom is buzzing.

What did the class discover?