Lesson Plan

• Describe how resistivity and geometry determine resistance in conductors.
• Explain the physical mechanism that causes an LDR’s resistance to fall as light intensity rises.
• Apply Ohm’s law and the power‑law relationship to calculate LDR resistance from measured voltage and illuminance.
• Analyse experimental data by plotting log‑log graphs to determine the light‑sensitivity exponent.

• Digital oscilloscope or multimeter (voltmeter)
• 5 V DC power supply
• LDR (CdS photoresistor) and a known reference resistor
• Lux meter
• Breadboard and connecting wires
• Projected diagram of LDR structure and sample data table

Begin with a quick demonstration: cover an LDR in a dark box and then expose it to a flashlight, asking students what they notice about the voltage reading. Connect this observation to prior knowledge of resistance and Ohm’s law. State that by the end of the lesson they will be able to predict and quantify how light changes resistance.

1. Do‑Now (5'): Students answer a short question on how temperature affects resistivity.
2. Mini‑lecture (10'): Review resistance, resistivity, and introduce LDR construction and the power‑law formula.
3. Demonstration (10'): Live circuit set‑up with LDR, reference resistor, and multimeter; show voltage change under varying light.
4. Guided Practice (15'): Students work in pairs to record voltage for three light levels, calculate resistance, and log lux values.
5. Data Analysis (10'): Whole‑class plot of log(R) vs. log(I) on the board; discuss slope and the exponent α.
6. Concept Check (5'): Quick exit ticket – one sentence explaining why LDR resistance drops with more light.

Summarise that LDR resistance inversely follows light intensity and that the relationship can be quantified using the power‑law. Students complete an exit ticket summarising the mechanism and receive a homework task to design a simple night‑light circuit using an LDR.