The correct option is **C. Mercury thermometer**. ### Detailed Explanation **1. Understanding Thermal Expansion:** Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature. When a substance is heated, its particles move more vigorously and tend to occupy a larger volume. This principle is crucial in the operation of certain types of thermometers. **2. How the Mercury Thermometer Works:** A mercury thermometer consists of a glass tube that contains mercury, a liquid metal. As the temperature increases, the mercury expands and rises in the tube. The height of the mercury column corresponds to the temperature, which can be read on a calibrated scale marked on the glass. - **Step-by-step operation:** - **Heating:** When the thermometer is exposed to a heat source, the temperature of the mercury increases. - **Expansion:** The mercury expands due to thermal expansion, moving up the narrow tube. - **Measurement:** The height of the mercury column is then read against the scale, providing a temperature reading. **3. Why Other Options Are Incorrect:** - **A. Bimetallic thermometer:** - A bimetallic thermometer operates on the principle of thermal expansion of two different metals that are bonded together. When heated, one metal expands more than the other, causing the strip to bend. This bending is then translated into a temperature reading. While it does involve thermal expansion, it does not use a liquid, which is a key aspect of the question. - **B. Digital thermometer:** - Digital thermometers do not rely on thermal expansion. Instead, they use electronic sensors (like thermocouples or thermistors) to measure temperature. These sensors convert temperature changes into electrical signals, which are then displayed digitally. Therefore, they do not operate on the principle of thermal expansion of a liquid. - **D. Infrared thermometer:** - Infrared thermometers measure temperature by detecting the infrared radiation emitted by an object. They do not involve any liquid or thermal expansion principles. Instead, they rely on the concept of radiation and the emissivity of surfaces to determine temperature. ### Common Pitfalls: - Confusing the principle of thermal expansion with other temperature measurement principles, such as electrical resistance or radiation. - Overlooking the specific requirement of a liquid in the question, which is a defining characteristic of the mercury thermometer. ### Summary: - The mercury thermometer operates on the principle of thermal expansion of a liquid (mercury). - Other thermometers like bimetallic, digital, and infrared do not use liquid expansion for temperature measurement. - Understanding the specific mechanisms of different thermometers is crucial for identifying their principles of operation. - Always pay attention to the details in the question to avoid confusion between different types of thermometers.