5.1.8Electricity and Magnetism

LEDs — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 6 parts Lesson 9 of 18

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Part 1 of 4What an LED is

A special diode

A special diode A light-emitting diode (LED) is a type of diode.
- Like any diode, it lets current pass in one direction only.
When current passes through it in that allowed direction, it gives out light.

Forward bias

Forward bias An LED lights up only when it is connected the correct way round, called forward bias.
If it is connected the wrong way round, it has a very high resistance, no current flows and no light is produced.

Needs a protective resistor

Needs a protective resistor An LED can be damaged if too much current passes through it,
- so a resistor is normally placed in series with it to limit the current.

LED

Indicators

Indicators Because an LED lights up when current flows, it shows that a circuit is switched on, for example the standby light on a television.

Lighting

Lighting LEDs are now used in home lighting, torches, street lamps, car headlights and display screens.

Efficiency

Efficiency An LED needs only a very small current.
- It transfers a larger fraction of the electrical energy to light than other types of lamp, so less energy is wasted as heat.

Long life

Long life LEDs run cooler than filament lamps and last much longer, so they are cheaper over their lifetime even if they cost more to buy.

Feature	Filament lamp	LED
How light is produced	Filament is heated until it glows	Current through a semiconductor junction releases light
Energy wasted as heat	Most of the input energy	Only a small fraction
Current needed	Relatively large	Very low
Direction of connection	Works either way round	Must be forward biased
Typical lifetime	Short	Long

1Start with the diode

Start with the diode A diode is drawn as a triangle pointing at a vertical bar.
- Current is allowed in the direction the triangle points.

2Add a circle

Add a circle The symbol is often placed in a circle; the circle is usually optional for components such as LEDs and LDRs,
- but not for meters and lamps.

3Check the arrows

Check the arrows Two arrows pointing away from the symbol mean it gives out light, so it is an LED.

4Compare with the LDR

Compare with the LDR Two arrows pointing towards the symbol mean the component is light dependent, so it is an LDR, not an LED.

Term	Meaning
Diode	A component that allows current to flow in one direction only.
LED	A light-emitting diode, which gives out light when current passes through it in the forward direction.
Forward bias	Connecting a diode so that current can flow through it.
LDR	A light-dependent resistor, whose resistance falls as light intensity rises.
Efficiency	The fraction of the input energy that is transferred to useful energy.

Mixing up the LED and LDR symbols. Arrows pointing away mean light is given out (LED); arrows pointing in mean it responds to light (LDR).

Saying that an LED works whichever way it is connected. It lights only when forward biased.

Claiming that LEDs use no energy. They use much less than other lamps, but they still transfer energy from the electrical store.

Leaving out the series resistor in a practical circuit. Without it too much current can destroy the LED.

Describing an LED as getting very hot to produce light. It is efficient precisely because little energy is wasted as heat.

Last updated: 20 October 2026