Why can I see colors in clear tape when held at a angle in front of a monitor? I took multiple strips of clear tape and stuck them together one after another to make a square tape mask. But when I put it in front of my laptop monitor at a angle, each individual strip of tape becomes colorful. (Red, green, blue etc.). Why does this happen?
 A: The light produced by your monitor is polarized, and the plastic film base of the adhesive tape has been stretched anisotropically during its manufacture. That stretching makes the film's index of refraction anisotropic, an effect called birefringence. Colored fringes result when polarized light is shined through a piece of birefringent material.
Before inexpensive computation, engineering stress analysis was performed on complex machine parts by replicating scale models of them in clear plastic known to exhibit birefringence, shining polarized light through them, and applying scaled stresses to them. The resulting strain distributions would show as colored fringes which could be correlated with the magnitude of the stresses.
A: If you put birefringent material between two polarizers, you will see colors.  However, this does require two polarizers.  The monitor qualifies as having a polarizer.  At first glance, the tape assembly you made does not include a polarizer per se.  However, if you tilt the assembly at an angle away from the monitor at around 57 degrees, light striking the surface reflects off, if it's polarized in one direction and is transmitted if it's polarized in the other direction, so now the assembly does act as a polarizer on both of its surfaces .  I think you may find that you can see the colors even without the monitor, if you just look at a bright light source through the assembly.  That 57 degree angle is called Brewster's angle, and is important in a lot of optics applications.
