Which device's resistance changes with light intensity?

Which device's resistance changes with light intensity?

• A. Filament bulb
• B. LDR
• C. Diode
• D. Wires

Answer: (B)

Light level changing the resistance is what an LDR is designed to do. An LDR, or Light-Dependent Resistor, is a semiconductor device whose conductivity rises as more light shines on it. Photons create additional charge carriers in the material, so when light intensity increases, the resistance drops. In darkness the material has very few carriers, so the resistance is high; in bright light the resistance becomes much lower, allowing more current to flow. This behavior makes LDRs ideal for light-sensing circuits, like automatic street lights or camera light meters. By comparison, a filament bulb changes its resistance mainly because its temperature rises as it glows, which isn’t a controlled response to light level, and a regular diode’s resistance isn’t governed by light (unless you’re using a photodiode). Wires have nearly constant resistance determined by material and dimensions, not by light.

Light level changing the resistance is what an LDR is designed to do. An LDR, or Light-Dependent Resistor, is a semiconductor device whose conductivity rises as more light shines on it. Photons create additional charge carriers in the material, so when light intensity increases, the resistance drops. In darkness the material has very few carriers, so the resistance is high; in bright light the resistance becomes much lower, allowing more current to flow. This behavior makes LDRs ideal for light-sensing circuits, like automatic street lights or camera light meters. By comparison, a filament bulb changes its resistance mainly because its temperature rises as it glows, which isn’t a controlled response to light level, and a regular diode’s resistance isn’t governed by light (unless you’re using a photodiode). Wires have nearly constant resistance determined by material and dimensions, not by light.