# A security light uses a light dependent resistor (LDR) connected in a circuit to switch a lamp on automatically at night. Explain how the LDR causes the lamp to switch on as it gets dark.

> OCR A-Level Physics A (H556) — 3.1 Motion · Explain · 5 marks

> Security lights that switch on automatically at dusk are common in homes. They rely on an LDR to detect changes in light intensity and trigger the lamp.

## Mark scheme (5 marks)

1. As it gets dark, the light intensity falling on the LDR decreases
2. The resistance of the LDR increases (as light intensity decreases)
3. This increases the total resistance of the circuit
4. So the current in the circuit decreases
5. The change in current (or potential difference across the LDR) is used to trigger / switch on the lamp when it gets dark enough

## Key terms

- [light dependent resistor (LDR)](https://www.gradenine.co.uk/glossary/light-dependent-resistor-ldr)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-security-light-uses-a-light-4380d006) · Published by Druglandscape Ltd.