A. Resistivity
B. Photoconductivity
C. Conductivity
D. Both (A) and (C)

Solution

B. Photoconductivity

Explanation

An LDR (Light Dependent Resistor) sensor works on the principle of photoconductivity. When light falls on the surface of the LDR, the resistance of the LDR changes. This change in resistance is inversely proportional to the intensity of light falling on it.

Here's how it works in more detail:

• Photoconductivity: LDRs are made of semiconductor materials whose electrical conductivity increases when exposed to light. When photons (light particles) strike the semiconductor material, they generate electron-hole pairs. These free charge carriers increase the conductivity of the material, leading to a decrease in resistance.
• Inverse Relationship: The resistance of the LDR decreases as the intensity of light increases, and vice versa. In other words, when there is more light, the LDR offers less resistance, and when there is less light, the LDR offers more resistance.
• Voltage Divider: LDRs are often used in conjunction with other electronic components, such as resistors, to create a voltage divider circuit. By measuring the voltage across the LDR in the voltage divider circuit, we can indirectly measure the intensity of light falling on it.
• Applications: LDR sensors are commonly used in various applications such as automatic street lights, brightness control in displays, photography light meters, and burglar alarms, where detecting changes in light intensity is required.