← IGCSE Computer Science Paper 1

LCD screen

Screen with a backlight shining through liquid crystals that twist to block or pass light. Red, green and blue filters on each pixel make its colour.

Part of: IGCSE Computer Science 0478 — 3.2 Input and Output Devices — Module 19

Understanding LCD screen in IGCSE Computer Science Paper 1

What it does: Light flows in order from the backlight, through the liquid crystal layer, then through the colour filters to the viewer. Monitors, TVs and laptops are common devices that use LCD screens to display images. Each pixel has red, green and blue filters. Mixing different amounts of these lights makes the pixel's colour.

Common mistakes: An LCD makes no light of its own. The backlight must be ON so light can shine through the liquid crystals. It should be backlight ← ON. The light that passes through each pixel comes from the screen's own backlight, not from room light. It should be light ← backlight.

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This module's key terms are also in a printable crossword (Modules 262–272) and word search (Modules 262–282).

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Learning
Duo
What lights up an LCD screen so you can see the picture?
A backlight shining through the panelEach pixel makes its own light
Matches
An LCD screen makes its own light from each pixel
↓
An LCD pixel does not make light itself. A backlight shines through the liquid crystals and the colour filters.
Applying
Meanings
Term
LCD screen
Meaning
Screen with a backlight shining through liquid crystals that twist to block or pass light. Red, green and blue filters on each pixel make its colour.
Practice
1 Backlight shines through the panel
Testing
Gaps
lightSource ←
Check Answer
Spots
1screen ← LCD
2backlight ← OFF

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Quick Questions

1. What lights up an LCD screen so you can see the picture?
A. A backlight shining through the panel ✓
B. Each pixel makes its own light
An LCD does not make light in the pixels. A backlight behind the panel shines through the liquid crystals and filters.
2. What do the liquid crystals in an LCD do?
A. Glow to give off their own light
B. Twist to block or let through light ✓
Liquid crystals twist to control how much backlight passes through each pixel. They do not produce light themselves.
3. Which colour filters does each LCD pixel use?
A. Red, green and blue ✓
B. Cyan, magenta and yellow
Each pixel has red, green and blue filters. Mixing different amounts of these lights makes the pixel's colour.
4. Which statement about an LCD screen is correct?
A. The backlight is always behind the liquid crystal layer ✓
B. The backlight is in front of the liquid crystal layer
The light has to shine from behind, through the liquid crystals and colour filters, towards the viewer.
5. How does an LCD pixel show a darker colour?
A. The backlight is removed from that one pixel
B. Crystals twist to let less light pass through the filter ✓
The backlight stays on for the whole panel. The liquid crystals twist to block more of the light for that pixel.

Match the Pairs

LCD screen → A screen where a backlight shines through liquid crystals that block or pass light to make each pixel's colour
RGB colour filters → Red, green and blue filters on each pixel that colour the light passing through the liquid crystals
The backlight shines through the panel, and the liquid crystals twist to let through more or less of that light → backlight -> liquid crystals -> colour filters
LCD screens are used in monitors, TVs and laptops → LCD = [monitor, TV, laptop]
An LCD screen makes its own light from each pixel → An LCD pixel does not make light itself. A backlight shines through the liquid crystals and the colour filters.
Liquid crystals in an LCD are the backlight → Liquid crystals are not the light source. They twist to control how much backlight passes through each pixel.

Key Term

LCD screen
Screen with a backlight shining through liquid crystals that twist to block or pass light. Red, green and blue filters on each pixel make its colour.

Fill in the Blanks

1. lightSource ← backlight
Hint: Type the name of the light that shines through the LCD panel from behind.
2. IF voltage = ON THEN crystals.twist() ENDIF
Hint: Type the method name for what the liquid crystals do to control the light.
3. filtersPerPixel ← 3
Hint: Type the number of colour filters each pixel has (red, green and one more).
4. FOR pixel ← 1 TO totalPixels mix(red, green, blue) NEXT pixel
Hint: Type the third colour filter on each pixel, after red and green.
5. lcdUses ← ["monitor", "TV", "laptop"] OUTPUT LENGTH(lcdUses)
Hint: Type the function that counts how many items are in the list of LCD uses.

Spot the Error

Question 1
A screen ← LCD
B backlight ← OFF
C light shines through crystals
Line B — An LCD makes no light of its own. The backlight must be ON so light can shine through the liquid crystals. It should be backlight ← ON.
Question 2
A FOR EACH pixel IN panel
B light ← room light
C NEXT pixel
Line B — The light that passes through each pixel comes from the screen's own backlight, not from room light. It should be light ← backlight.
Question 3
A uses ← monitors, TVs, speakers
B backlight ← ON
C crystals twist per pixel
Line A — LCD screens are used in monitors, TVs and laptops. A speaker is an output device that makes sound, so it has no LCD screen.
Question 4
A FOR EACH pixel IN backlight
B set red/green/blue filters
C NEXT pixel
Line A — The pixels are part of the LCD panel (the screen), not the backlight. The loop should be FOR EACH pixel IN screen.
Question 5
A backlight ← ON
B light shines through crystals
C crystals twist: block or pass
D pixel colour ← white filter
Line D — Each pixel gets its colour from red, green and blue filters, not a single white filter. It should be pixel colour ← RGB filters.

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