There are three ways that objects can have thermal interactions with other objects.The most common method is by heat transmission. This happens when two of her objects with different temperatures come into contact and thermal energy is transferred from the warm object to the cold object, like when you hold a can of cold soda in your hand. The can is warm and your hands are cold.
The following heat transfer methods are convection, and it works only for gases and liquids. Let’s use air as an example. Suppose you have a heat source like a stove. The air near the stove burners heats up due to heat transfer interactions. This hot air is less dense than the cold air above it. The temperature rises and cold air flows in instead. Second, the hot air can interact thermally with anything above it, such as the ceiling. The indirect transfer of heat from the stove to the ceiling is convection.
A third type of thermal interaction is radiation— and this is what we really want. When a hot object emits infrared radiation, the radiation can be absorbed by other objects. This is exactly how the oven works. When you put something you want to cook inside, the heating element becomes very hot and produces thermal radiation. (Yes, it’s the same as infrared radiation.) Food absorbs this and raises its temperature.
Now imagine you preheated the oven, turned it off, and put the potatoes in it. A hot oven gives off heat radiation, and potatoes absorb most of that heat. As a result, the potatoes are hot and the oven is cold. This isn’t really the normal way to bake potatoes, but the point is that the object will cool as it produces heat radiation.
But if everything around us emits infrared electromagnetic radiation, we shouldn’t all Is it getting cooler? not much. If you put an apple on the table, it will emit heat radiation.but it is also absorb Radiation from everything else: table, air, walls. Therefore, if all objects in the same vicinity already have the same temperature, they will not be cooled by radiation.
reflectance and emissivity
There is another very important property to consider in order to fully understand how radiative cooling works. That is the difference between reflectance and emissivity. Imagine you have a perfect mirror. All light that hits it is reflected. That mirror will have a reflectance of 1. This means that 100 percent of the light hitting the mirror is reflected.
A sheet of aluminum foil also reflects quite a bit of light, but it doesn’t. all light. The reflectance is about 0.88, which means 88% is reflected. The remaining 12% of the light that hits the foil is absorbed, increasing the temperature of the foil.
Now imagine an object that reflects no light at all. Of course, it still emits light, but that’s because of its temperature, not because it’s reflecting light. This object has an emissivity of 1 and is called a “perfectly black body”, meaning that it absorbs all electromagnetic radiation. Therefore, emissivity is essentially the inverse of reflectivity.
