# Polarisation & LCDs: How Does Darkening On Turn On Work?

• vickyc
In summary, polarised light is used to light up an LCD screen by passing through a twisted polaroid filter. The display darkens when the liquid crystal blocks the reflected light, which is why the numbers show up as dark when the calculator is turned on. There is no backlight on simple LCD displays, but there is a mirror on the back that allows for the reflection of room light.
vickyc
I think I understand how polarised light is used to light up an LCD screen. The polarised light twists as it passes through the LCD, then goes through another polaroid filter which allows twisted light through to light up the display - unless there's a current in the electrodes, which means that the crystals don't twist the light. Is that right?

My question is: what about displays which darken when you turn them on? I have a calculator which is a kind of pale greeny non-colour turned off, but when I turn it on and start typing numbers in, the numbers show up as dark. How does that work?

vickyc said:
My question is: what about displays which darken when you turn them on? I have a calculator which is a kind of pale greeny non-colour turned off, but when I turn it on and start typing numbers in, the numbers show up as dark. How does that work?
There is no backlight on simple LCD displays but there is a mirror on the back.
With the liquid crystal in one state you can see the room light reflected from the back, in the other state the the liquid bocks the reflected light - hence black.

Yes, you are correct in your understanding of how polarised light is used in LCD screens. The liquid crystals in the display twist the polarised light, and then the second polaroid filter allows the twisted light to pass through and create the image on the screen.

As for displays that darken when turned on, this is due to the presence of a backlight in the LCD screen. When the display is turned off, the liquid crystals are in a relaxed state and the backlight is able to pass through easily, creating a pale color on the screen. However, when the display is turned on and a current is applied to the electrodes, the liquid crystals twist and block some of the light from the backlight, creating a darker image. This is why the numbers on your calculator appear dark when the display is turned on and a current is applied.

I hope this explanation helps clarify how the darkening effect works in LCD screens. Additionally, the amount of current applied to the electrodes can also determine the level of darkness on the screen, allowing for different levels of brightness in the display.

## 1. How does polarisation work in LCDs?

In LCDs, polarisation is used to control the amount of light that passes through the liquid crystal layers. This is achieved by aligning the molecules of the liquid crystals in a specific orientation, which can be controlled by applying an electric field.

## 2. What causes the darkening effect in LCDs when they are turned on?

The darkening effect in LCDs is caused by the alignment of the liquid crystal molecules in response to the electric field. When the electric field is applied, the molecules twist, causing the polarised light to be blocked and creating the dark appearance on the screen.

## 3. How does darkening on turn on work in LCDs?

The darkening effect in LCDs when they are turned on is achieved by using a polariser on the front of the screen and a second polariser on the back. The first polariser blocks all light except for horizontally polarised light, which is then twisted by the liquid crystal layer. The second polariser then blocks this twisted light, resulting in the dark appearance.

## 4. Can the darkening effect in LCDs be adjusted?

Yes, the darkening effect in LCDs can be adjusted by varying the strength of the electric field. This can be done by changing the voltage supplied to the liquid crystal layer, allowing for different levels of light to pass through and adjusting the brightness of the screen.

## 5. What are the advantages of using LCDs with polarisation?

Using polarisation in LCDs allows for greater control over the amount of light passing through, resulting in better contrast and image quality. Additionally, LCDs with polarisation consume less power compared to other display technologies, making them more energy efficient.

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