Department of Physics and Materials Science

Magic Glasses

What? Where Did this Come From?

The magic is actually in the physics underlying the liquid crystal display (LCD) used in this monitor!

LCD's combine a white backlight, an electrode grid, a liquid crystal layer, a set of color filters, and two polarizers.

1. The backlight is polarized by the rear polarizer (see The Polariscope display for more information).
2. The voltage applied by the electrodes changes the orientation of the liquid crystals, rotating the incoming light's polarization direction*
   1. Low voltage: 90° rotation; front polarizer blocks the light, forming black spot
   2. High voltage: no rotation (0°); front polarizer lets light pass freely, forming white spot
   3. In between: rotation between 0° and 90°; only a fraction of the light passes through the front polaroizer, forming a gray spot
3. Different voltages can be applied to each point on the screen, producing an image of black white and gray pixels.
4. Color is introduced by passing the light through filters sandwiched between the polarizers.

So what is special about this monitor? Its front polarizer has been removed, so no light is blocked, and no image is formed. That is, until you put on an ordinary pair of polarized sunglasses, which use a polarizing coating to block a portion of the Sun's light from reaching your eyes! You may also notice that tilting your head causes the colors to invert. With the sunglasses at this angle, the light that was blocked can now pass, while the light that could pass, while the light that could pass is now blocked!

 * The polarizer orientationsand the voltage required for a bright or dark spot depends on the type of LCD panel (TN, VA, or IPS). We describe the IPS configuration here.