LCD's are generally used in watches, calculators, lab-top computer screens and in many small size instruments that need to display some data. Now that we know the basic mechanism of an LCD let us take a closer look at it. Let's see in detail how an LCD clock works.

A liquid crystal clock as well as all kind of common displays are called TN (Twisted Nematic) displays. The name comes from the particular orientation of the rod-like molecules in the liquid crystal.
They are organized in such a way that their orientation smoothly changes of a 90 degree angle while going from the first to the second polarizer.

Orientation of the liquid crystal molecules in a twisted nematic display.

The two polarizers are crossed and have their transmission axis parallel to the director of the rods that stick on them. When light coming from outside passes through the first polarizer it gets polarized along the same direction of the molecules sticking on the polarizer. While traveling inside the liquid crystal its polarization direction smoothly changes according to the orientation of the rods. On the way out its polarization axis has undergone a total twist of 90 degree and it is now parallel to the transmission axis of the second polarizer which allows the light to make it through it.

Let's now suppose that we impose an external electric field in a direction perpendicular to the planes of the polarizers.
If the field is strong enough the rods inside the liquid crystal (due to their dipole) will be forced to orient themselves along the direction of the field. The particular process of preparation and the viscosity of the liquid crystal make it easier for the molecules in the middle of the sample to reorientate respect to those which are closer to the polarizers.

Molecules in a TN display try to orientate themselves along the inducting electric field direction.

The net result of this effect is that the rods which stick to the polarizers basically do not move and the rods in the middle completely change their orientation destroying the previous twisted geometry. Light passing through the liquid crystal in which the external electric field has been applied will not have its polarization axis twisted of 90 degree and will be stopped at the second polarizer. In other words, if you look through the display we just imagined, it will look transparent (the incident light can make it through it) in all the sectors in which the electric field is switched OFF and it will be black (the incident light can not make it through it) where ever the electric field is switched ON.