Thermal Radiation

What is Thermal Agitation in Physics?

The temperature of an object actually measures the degree of agitation of the molecules that compose it. The more an object is hot, the more its molecules are agitated by rapid and erratic movements; this is called the thermal agitation. It is said that an object is at a temperature of absolute zero when its particle can’t move at all.

But the matter is made with many charged particles (protons, electrons in atoms). So when they are agitated, they create disturbances of the electromagnetic field, i.e. they emit light. So any object whose temperature is not the absolute zero emits light continuously.

The light emitted by an object is characterized by its color (frequency) and its intensity. These two features are related to the temperature of the object. The more it is hot, the more its molecules move and the quicker they move. Therefore the more the frequency of light is large, the more its color will tend on the side of the blue and the more it will emit light.

Read More: What is Cosmology?- Cosmology Definition, Terms, Amazing Facts & Theories

An object at room temperature (say 20°C), whose molecules are not very rough, emits at a frequency low enough: they are infrared, we cannot see them. But there are cameras that see infrared rays and turn them into images on a screen. We can see hot areas brighter, as the body of someone. That’s how we can see in the dark, since even in complete darkness, objects, even if they are black, emit infrared.

thermal agitation
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Thermal Agitation examples

But for very hot objects (called incandescent), the light becomes visible. Consider some examples:

  • The filament of a bulb emits light because it is heated by the passage of electric power. This is called an incandescent light bulb, and it makes a fairly white light.
  • The surface of the molten lava is not as hot as the filament of the bulb. It also shines because it is very hot, but it does the red.
  • The stars have different colors, from red to blue, because they have different temperatures. The hottest are blue or white, while the “coldest” are red. We can know the temperature of a hot item just by looking at the light it emits.
  • This property is also used for heating the radiant panels that emit infrared. We do not see them, but we feel them: when the infrared comes to you, they will communicate their energy, and you warm. A classic radiator heats the air. The radiant panels are more effective to warm you, because even if you’re away, they will send infrared, without warming the air too much.
  • When you put your hands near the fire, it is also because you feel the infrared that you warm. There is no hot air coming up to you. The warm air is especially over the fire. If you feel the heat, the infrared!

Infrared is divided into near IR (NIR: 0.78 μm to 1.4 μm), mid IR (MIR: 1.4μm to 3 μm) and remote IR (from 3 μm to 1 000 μm). Although, the classification is not universal and its boundaries vary from one to another.

Recommended books to understand it better is:

  1. Optics

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2. Principles of Optics

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