3.20 Resistance & Illumination

• Light can cause a change in conductivity of some semi-conductors
• When light is absorbed by the material it causes more electrons to be available for conduction

• An increase in the number of conduction electrons reduces the resistance

• A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
• Its resistance automatically changes depending on the light energy falling onto it (illumination)
• As the light intensity increases, the resistance of an LDR decreases

Resistance of an LDR depends on the light intensity falling on it

• This is shown by the following graph:

Graph of light intensity and resistance for an LDR

• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
  • In the dark, its resistance is very large (millions of ohms)
  • In bright light, its resistance is small (tens of ohms)

LDRs are used for automatic street lights

Step 1: Consider the relationship between light intensity and resistance

• • As light intensity increases, resistance decreases in an LDR
  • If the resistance decreases then the potential difference will increase

Step 2: Consider a relevant equation

• • Ohm’s law states that V = IR
  • The resistance is equal to V/I or 1/R = I/V = gradient of the graph
    • Since R decreases, the value of 1/R increases, so the gradient must increase

Step 3: State the conclusion

• • Therefore, I increases with changing V with an increasing gradient
  • This is seen in graph A