Photon Remote Temperature Sensor
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Measuring Temperature
In our macroscopic world, temperature is a seemingly simple concept -- we intrinsically understand fundamental differences between hot objects and cold objects. Often we can estimate temperature just by looking at the object; if our stove burner is glowing red, we know that it is likely very hot and we should avoid touching it.
The standard definition of temperature is a measure of the average amount of heat, or thermal energy, of the particles in a substance. Since temperature is an average measurement, typically the size of the sample is irrelevant. For example, a small pot of boiling water will be at the same temperature as a large pot of boiling water (as long as you're at the same altitude!).
On the microscopic scale, temperature is a fairly complex phenomenon that corresponds to the molecular speed of individual atoms. In other words, temperature is a measure of the kinetic energy of the atoms that make up a particular substance. In general, for a given substance, the solid phase will have the lowest temperature because the atoms are mostly rigid, followed by the liquid phase in which atoms have more kinetic energy, and then the gaseous phase with the most energetic atoms. However, since mass is also a form of energy, less massive molecules need to have more kinetic energy to be at the same temperature as more massive molecules. This is why all three phases can exist at the same average temperature (e.g. gaseous oxygen, liquid water, solid metal).
Want to learn more about temperature? Check out this great resource.