Resistors: The resistance of an LDR decreases as light intensity increases.

The idea

A light dependent resistor, or LDR, responds to light rather than heat: the brighter the light falling on it, the lower its resistance. In darkness its resistance is very high, often hundreds of thousands of ohms, while in bright light it can fall to a few hundred. That makes it a light sensor. In a circuit the consequence again follows from V = I R. Brighter light lowers the resistance of the LDR, which lowers the total resistance of the circuit, so the current from the supply increases. The response is not linear, and the resistance changes over several orders of magnitude, which is why LDR circuits are usually drawn with a fixed resistor beside them to give a workable output.

What earns the marks

• Stating that the resistance of an LDR decreases as the light intensity increases.
• Deducing the effect on the total resistance of the circuit.
• Concluding whether the current increases or decreases.
• Interpreting a resistance against light intensity graph correctly.

Common mistakes and how to put them right

• Reversing the relationship and saying resistance rises in brighter light.

• Confusing light intensity with the colour or wavelength of the light.

• Assuming resistance halves when light intensity doubles, treating a curve as a straight line.

• Mixing up the LDR with the thermistor when a circuit contains both.

Exam tips

• Fix the two extremes in your memory: dark means high resistance, bright means low resistance.
• Follow the same chain as for a thermistor: resistance, total resistance, current.
• Read values from a resistance against intensity graph rather than estimating, since the scale is often unequal.