Linearly polarized light of LCD
Experiment number : 4301
Goal of experiment
The goal of the experiment is to demonstrate the properties of linearly polarized light when it passes through a polarizing filter.
Theory
A liquid crystal display (known as LCD) is a source of linearly polarized light and can be easily used in teaching wave optics. If we place a linear polarizing filter in front of the display and rotate it in a plane parallel to the screen, we will see that in some positions the filter transmits all the light from the display, while when rotated 90° all the light is absorbed by it. The first case corresponds to the plane of the polarized light being identical with the plane in which the filter transmits it, while in the second case these planes are perpendicular to each other.
Tools
LCD monitor, polarizing filter.
Procedure
Place a linear polarizing filter in front of the LCD monitor and rotate it in a plane parallel to the screen. Observe the gradual darkening of the image until the monitor goes completely dark; on further rotation, it starts to light up again.
Sample result
The video shows the experiment.
Technical notes
If you try the above experiment with smartphone displays, you’ll find that many of them don’t respond much to rotation with a linear polarizing filter. These displays are no longer made with LCD technology but with OLED (organic light emitting diodes) technology and typically produce circularly polarized light rather than linearly.
Methodological notes
Students are very familiar with the terms LCD TV and LED TV and often assume that they are two completely different technologies. In reality, LED televisions are a “subset” of LCD televisions – they are a simple LCD display in which the backlighting is done by light emitting diodes (hence LED). This backlighting method has now completely displaced the cold cathode backlighting used in the previous generation of LCD displays.
In contrast, so-called OLED TVs, which do not use a backlight, work in a fundamentally different way.
How a liquid crystal display (LCD) works
Although the foundations of liquid crystal display (LCD) technology date back to the 19th century, the first LCD (liquid crystal display) screen was produced in 1968, and did not become more widespread until the 1990s.
The back side of the LCD screen is made up of light sources – previously cold cathode lamps (CCFL), nowadays light emitting diodes (LED) are used. The light then passes through a horizontally oriented polarizing filter into a liquid crystal layer. The liquid crystals are made up of helical molecules which, under normal circumstances, turn the polarization angle by 90°. However, when an electrical voltage is applied to them, these molecules orient themselves in the direction of the electric intensity vector of the incident light and do not rotate the plane of polarization.
Behind the liquid crystal layer, there is usually a layer of colour filters to determine the pixel colour, and another polarizing filter, this time vertical. If the light has passed through the liquid crystal layer with an applied electrical voltage, then its plane of polarization has not changed and it will not pass through the vertical filter. However, if it passed through a layer of liquid crystals with no applied electrical voltage, the plane of polarization has been twisted by 90\(^\circ\) and the light will pass through the vertical filter.
This principle is explained very clearly in a video posted on the In One Lesson channel: In One Lesson:
