Why does an LCD need a backlight or a reflector?

Liquid crystals do not produce light and only control light passing through them. A backlight or a mirror-like reflector behind the display provides the light that is seen.

Why are LCDs used in calculators and digital watches?

LCDs use very little power, so a small battery or solar cell lasts a long time. A reflective LCD can be read in daylight without any backlight.

What is a seven-segment LCD, and what can it show?

A seven-segment LCD has seven bar-shaped segments arranged in a figure of eight. Different combinations of darkened segments form the digits 0 to 9 and some letters.

CCEA A-Level Technology and Design 8900

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Composite and Emerging Materials

Liquid crystal displays

3 things to know
AS 1 Paper 1

Overview

Liquid crystals and twisted nematic LCDs, backlights and reflectors, seven-segment and dot matrix LCDs, colour LCDs, their advantages and limitations, and LCD against LED displays.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a liquid crystal?

Key term

What is a liquid crystal display?

Key term

Explain how a segment of a twisted nematic LCD turns dark.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a liquid crystal?
Model answer
A liquid crystal is a substance that flows like a liquid but has molecules lined up like a crystal. The alignment of the molecules can be changed by an electric field.
2
What is a liquid crystal display?
Model answer
A liquid crystal display holds a thin layer of liquid crystal between two polarising filters. The display controls the light passing through each segment or pixel.
3
Explain how a segment of a twisted nematic LCD turns dark.
Model answer
Twisted liquid crystal molecules rotate polarised light so that it passes through the second filter when no voltage is applied. A voltage untwists the molecules, so the light is blocked and the segment looks dark.
4
Why does an LCD need a backlight or a reflector?
Model answer
Liquid crystals do not produce light and only control light passing through them. A backlight or a mirror-like reflector behind the display provides the light that is seen.
5
Why are LCDs used in calculators and digital watches?
Model answer
LCDs use very little power, so a small battery or solar cell lasts a long time. A reflective LCD can be read in daylight without any backlight.
6
What is a seven-segment LCD, and what can it show?
Model answer
A seven-segment LCD has seven bar-shaped segments arranged in a figure of eight. Different combinations of darkened segments form the digits 0 to 9 and some letters.

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