Assertion–Reason on microwave power measurement with a bolometer Assertion (A): A bolometer is frequently used for microwave power measurement. Reason (R): In a bolometer, the incident microwave power causes a measurable change in resistance.

Introduction / Context:
Measuring RF/microwave power accurately often relies on thermal techniques that convert RF energy into heat. Bolometers and thermistors are classic sensors in power meters, especially where broadband, calibration-friendly methods are required. This assertion–reason problem asks whether the stated mechanism correctly explains the common use of bolometers in microwave power metrology.

Given Data / Assumptions:

• Bolometer element exhibits resistance that varies with temperature.
• Microwave power absorbed in the load produces heating.
• A bridge or feedback circuit tracks resistance change to infer power.

Concept / Approach:
Bolometric power measurement is fundamentally calorimetric: RF energy is dissipated, raising the sensor temperature and changing its resistance. In a substitution method, DC power is adjusted to reproduce the same resistance change, and the equivalent RF power is read from the required DC level. This mechanism directly justifies the frequent use of bolometers for accurate, broadband power measurement at microwave frequencies.

Step-by-Step Solution:

Verification / Alternative check:
Standards labs have long used bolometric and thermistor mounts traceable to calorimetric principles; modern sensors (thermopile, diode) still rely on calibrated transfer to a thermal equivalent.

Why Other Options Are Wrong:

• B: The reason is directly causal, not incidental.
• C/D: Both incorrectly deny either the accuracy of the assertion or the validity of the mechanism.

Common Pitfalls:
Confusing diode sensors (rectifying detection) with purely thermal bolometers; overlooking the substitution method that makes calibration straightforward.

Final Answer:
Both A and R are correct and R is correct explanation of A