# A street lighting system uses light dependent resistors (LDRs) to automatically switch lights on at night and off during the day. Explain how the resistance of an LDR changes between daytime and night-time, and explain why this makes LDRs suitable for use in automatic street lighting systems.

> OCR A-Level Physics A (H556) — 3.2 Forces in action · Explain · 5 marks

> Modern street lights often use sensors rather than timers to control when lights turn on and off. One common type of sensor relies on how certain components respond to changes in light intensity.

## Mark scheme (5 marks)

1. The resistance of an LDR decreases as light intensity increases (daytime = lower resistance)
2. The resistance of an LDR is greatest when it is dark / at night-time
3. The change in resistance of the LDR causes a change in current in the circuit
4. This changing current (or signal) can be used to trigger the lights to switch on when it gets dark
5. The process is automatic / the LDR responds directly to light intensity without human input, making it a suitable sensor for this application

## Key terms

- [resistance](https://www.gradenine.co.uk/glossary/resistance)

## 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-street-lighting-system-uses-light-09c7785c) · Published by Druglandscape Ltd.