Start your study
of physics by examining vision
. . . and especially
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.
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...
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.
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."
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.
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.
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."
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."
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
color was it?"
of hesitation. Then a tentative, "Green?" The instructor takes
the green book and repeats the experiment.
says the subject. Back to the rear goes the book. But the color
name doesn't come easily. . . "Blue?"
you sure you're not colorblind?"
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.
suggestions?" says the instructor.
says, "Peripheral vision is colorblind."
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.
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."
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."
the classroom is buzzing.
What did the class discover?