Controlling Light with Electric Fields
I guess for the light parts of the display, you'd just leave the liquid
crystal alone so the light goes right through.
That's right. Between the two perpendicular filters, the twisted cells are
arranged to rotate the light's polarization exactly 90 degrees...
Then how do you make the numbers--the dark areas, I mean? Can you do something
to the molecules so that they no longer rotate the light?
Yes, you can. Remember, these molecules are able to affect the light's
polarization because of their special shape and arrangement. Imagine that you
could reach into the crystal and tilt the molecules so that the light hits
them all head-on. Now instead of "seeing" a bunch of long french fries
arranged in a helix, the incoming light just "sees" their small
...and so the light isn't twisted! But how do you tilt the molecules?
Do you remember how electric
forces can move a water molecule?
Yeah, they can rotate it, because the water molecule has a positive end and a
Yes. In the same spirit, we can use an
electric field to tilt the liquid crystal molecules.
So that electric field was the "signal" in that demo you showed me--and that's why the cell was
dark when the signal was on.
Exactly. The applet below shows a side view of the twisted cell; it lets you
see exactly how electric fields affect the molecules. Notice how the
orientations of the molecules change as you move from left to right.