Lesson Three : Thermal Radiation
The most familiar form of electromagnetic radiation is thermal radiation. We observe thermal radiation all the time. Place your hand on your skin and you will feel the warmth of the radiation from your body. If an object emits enough radiation (such as a hot stove), the radiation becomes visible and you will observe a bright red glow in addition to the heat.Temperature determines the brightness and intensity of radiation that an object emits. If an object is hotter, the electromagnetic radiation it produces will be more energetic. According to Wien’s Displacement Law, a hotter object will emit radiation with a shorter wavelength. For instance, with objects emitting visible light, hotter objects will glow bluer, since blue light has a shorter wavelength and more energy.
For example, when the heating element on a stove is activated it emits infrared radiation that we perceive as heat. It is also emitting radio waves. As the burner gets hotter, however, it begins to glow and become visible. The burner is now emitting red light, which has a shorter wavelength than infrared light.
According to the Stefan-Boltzmann Law, hot objects will also emit more energy over time. The total energy emitted every second by a section of the object will increase proportional to the temperature of the object. The Stefan-Boltzmann law provides a technical description of a very familiar phenomenon. A hotter object pumps out more energy per second than a cooler object. If, for example, the heating element mentioned above, which is already emitting a dull red light, is heated even further, it will begin to glow bright red. Hotter objects will glow brighter.
In summary, thermal radiation is governed by the following two laws:
- Hotter objects emit light with a shorter wavelength and a higher frequency.
- Hotter objects emit brighter light.