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The Optical Pyrometer (also called the disappearing filament pyrometer) has a specific operational temperature range. Option A (200°C to 1000°C) is incorrect — this range is too low for the Optical Pyrometer. At temperatures below 800°C, the visible light radiation from a body is insufficient for the disappearing filament method to work accurately. Option B (500°C to 1500°C) is incorrect — while the upper end (1500°C) is within the optical pyrometer's range, the lower end (500°C) is incorrect. The minimum operational temperature for Optical Pyrometers is 800°C because only at this temperature does a body begin to emit sufficient visible radiation for optical comparison. Option C (800°C to 2700°C) is CORRECT — the Optical Pyrometer can measure temperatures from 800°C to 2700°C. The lower limit is 800°C because that is when visible incandescence begins. Option D (1000°C to 5000°C) is incorrect — the upper limit of Optical Pyrometers is 2700°C, not 5000°C.
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Heat is a form of energy that transfers between objects or systems due to a temperature difference, moving from a hotter body to a cooler one until thermal equilibrium is reached.
