6.3.2 Perfect Black Bodies and Radiation

In this lesson, we will explore the concept of radiation emitted by all bodies and how the intensity and wavelength distribution of this emission depend on the body's temperature.

Emission of Radiation

All bodies, regardless of their composition, emit radiation. This process is known as thermal radiation.

• The intensity and wavelength distribution of the emitted radiation depend on the temperature of the body.
• Hotter bodies emit more intense radiation, with a distribution that peaks at shorter wavelengths.

Absorption and Emission Equilibrium (HT only)

A body at a constant temperature absorbs radiation at the same rate as it emits radiation, establishing an equilibrium. When a body absorbs radiation faster than it emits radiation, its temperature increases.

Factors Influencing Earth's Temperature (HT only)

The temperature of the Earth is determined by various factors, including:

• Rates of Absorption and Emission: The balance between the incoming radiation absorbed from the Sun and the radiation emitted by the Earth affects its overall temperature.
• Reflection of Radiation: The reflection of solar radiation by the Earth's surface and atmosphere plays a role in controlling the amount of energy absorbed.

Temperature and Radiation Balance (HT only)

The temperature of a body is related to the balance between the incoming radiation absorbed and the radiation emitted.

• Everyday Examples: Examples from everyday life include a cup of coffee cooling down or a warm object reaching room temperature.

Radiation and Earth's Temperature (HT only)

• Solar Radiation: The Sun emits electromagnetic radiation, with a significant portion falling within the visible and infrared spectra, which interacts with the Earth's atmosphere and surface.
• Greenhouse Effect: Certain atmospheric gases, known as greenhouse gases, absorb and re-emit infrared radiation, trapping heat in the Earth's atmosphere and contributing to the overall temperature rise.

Conclusion

All bodies emit radiation, and the intensity and wavelength distribution of this emission depend on the body's temperature. A body at a constant temperature absorbs radiation at the same rate as it emits radiation, maintaining an equilibrium. The temperature of a body is determined by the balance between incoming radiation absorbed and radiation emitted.