understand that the resistance of a light-dependent resistor (LDR) decreases as the light intensity increases

Resistance and Resistivity: Light‑Dependent Resistor (LDR)

An LDR (Light‑Dependent Resistor) is a special type of resistor whose resistance changes with the amount of light that falls on it.



Key idea: More light → lower resistance (the opposite of a normal resistor).

Why does light affect resistance?

In a semiconductor, light creates electron‑hole pairs.

These extra charge carriers make it easier for current to flow, so the material becomes more conductive (lower resistance).

Think of it like adding more lanes to a highway: traffic (current) moves faster and the “traffic jam” (resistance) lessens.

Basic Equations

  • Ohm’s Law: \$V = IR\$

  • Resistance of a uniform material: \$R = \dfrac{\rho L}{A}\$

  • LDR behaviour: \$R \propto \dfrac{1}{I_{\text{light}}}\$ (roughly)

Typical LDR Response

Light Intensity (lux)Resistance (kΩ)
0 (dark)>10 kΩ
50 lux (dim)5 kΩ
500 lux (bright)1 kΩ
10 000 lux (very bright)0.1 kΩ

Practical Example: Night‑Vision Camera

In a night‑vision camera, an LDR is used to adjust the brightness of the image automatically.

When the scene is dark, the LDR’s high resistance limits current to the sensor, preventing over‑exposure.

As light increases, the resistance drops, allowing more current and brightening the image.

🚦 Analogy: Think of the LDR as a smart dimmer switch that reacts instantly to the room’s lighting.

Exam Tip Box

Remember:

  1. State the relationship: Resistance decreases as light intensity increases.

  2. Use the proportionality \$R \propto 1/I_{\text{light}}\$ when sketching graphs.

  3. Explain the physical reason: Light generates charge carriers in the semiconductor.

  4. Include a simple diagram or sketch of the LDR circuit if the exam allows.

Quick Quiz (for self‑check)

  1. What happens to the resistance of an LDR when it is placed in a bright spotlight?

  2. Write the equation that relates resistance to resistivity, length, and cross‑sectional area.

  3. Why is an LDR useful in automatic street lighting systems?

Answer key:

1. Resistance decreases (becomes lower).

2. \$R = \dfrac{\rho L}{A}\$

3. It automatically reduces current when the street is bright, saving energy, and increases current when it gets dark, ensuring safety. 🚦