Describe how the resistance of a light-dependent resistor (LDR) varies…

The idea

An LDR is a resistor whose resistance depends on the light falling on it. In darkness its resistance is high, so only a small current flows for a given applied voltage. As light intensity increases, more charge carriers are released in the semiconductor material, so the resistance falls and the current rises. The relationship is not proportional: the resistance drops steeply at low light levels and then changes more slowly at high intensity. A useful way to remember the direction is that bright light gives a low resistance, while darkness gives a high resistance. This makes an LDR suitable for sensing light level, for example in a circuit that switches a lamp on when it gets dark. When answering, state the direction of change clearly and link it to light intensity rather than to voltage or temperature.

What earns the marks

• States that resistance is high when light intensity is low or in darkness.
• States that resistance falls as light intensity increases.
• Links the change in resistance to the number of charge carriers released by light.
• Recognises that the resistance change is not directly proportional to light intensity.
• Applies the idea to a sensing circuit, such as a lamp switching on in darkness.

Common mistakes and how to put them right

• Claims that resistance rises as light intensity increases.

  Put it right by stating that bright light lowers the resistance.

• Treats the change as directly proportional.

  Put it right by noting the resistance falls steeply at low light and levels off at high light.

• Confuses the LDR with a thermistor.

  Put it right by linking the LDR response to light intensity, not temperature.

Exam tips

• Name the input quantity first (light intensity) and then state the direction of the resistance change.
• Use the words high and low resistance rather than vague terms such as big or small.
• Add a brief reason: light releases charge carriers, so resistance falls.