Perfect Black Bodies

A PERFECT BLACK BODY is an object that:

• ABSORBS all INCIDENT radiation (radiation that lands on it).
• Doesn't REFLECT or TRANSMIT any incident radiation.
• EMITS (gives off) the most amount of radiation possible.

Since it absorbs all radiation that falls on it, it must re-emit all the energy it absorbs, adhering to the principle of conservation of energy.

If a body EMITS energy at the SAME RATE that it ABSORBS it, it stays at a CONSTANT TEMPERATURE. When this occurs, we say that the body is in THERMAL EQUILIBRIUM.

The temperature of the body INCREASES when it ABSORBS radiation FASTER than it EMITS it.

Temperature Effects on Emission

All bodies (objects) emit a SPECTRUM of thermal RADIATION in the form of ELECTROMAGNETIC waves.

The INTENSITY and WAVELENGTH distribution of any emitted waves depends on the TEMPERATURE of the body.

This is represented on a black body radiation curve:

As the temperature increases, the PEAK of the curve:

1. Gets HIGHER which shows that the PEAK INTENSITY of the radiation emitted INCREASES.

2. Moves to the LEFT which shows that the PEAK WAVELENGTH emitted DECREASES.

Earth’s Temperature and Infrared Radiation

The temperature of the Earth is determined by the balance between INCOMING infrared radiation from the Sun and the outgoing infrared radiation. This balance involves several processes:

ABSORPTION: The Earth absorbs solar radiation, which HEATS the planet.

EMISSION: The Earth emits infrared radiation back into space, which COOLS it down.

REFLECTION: Some of the solar radiation is reflected back into space by the Earth’s surface and atmosphere.