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The Total Radiation Pyrometer is used for measuring very high temperatures using the total energy radiated by a body. Option A (Wien's displacement law) is incorrect — Wien's displacement law relates the peak wavelength of radiation to the absolute temperature. Wien's law is used by Optical Pyrometers, not Total Radiation Pyrometers. Option B (Newton's law of cooling) is incorrect — Newton's law of cooling describes the rate of heat loss from a body to its surroundings due to convection. Option C (Snell's law) is incorrect — Snell's law describes the refraction of light at an interface between two media. It is used in optics (lenses, prisms) but has no relation to radiation pyrometry. Option D (Stefan's law) is CORRECT — the Total Radiation Pyrometer is based on Stefan-Boltzmann law (Stefan's law): E = σT⁴. The pyrometer collects all the radiation (total, not wavelength-specific) from a hot body and focuses it on a thermocouple or bolometer. By measuring the total energy received, and using Stefan's law, the temperature is calculated. Range: 800°C to 3000°C — slightly wider than Optical Pyrometer.
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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.
