6.3 Black body radiation Flashcards
Black body radiation
Black body radiation is the name given to the thermal radiation emitted by all bodies (objects).
All objects, no matter what temperature, emit black body radiation in the form of electromagnetic waves.
The hotter object, the more infrared radiation it radiates in a given time.
Perfect black body
An object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation.
Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter too.
As a result, an object which perfectly absorbs all radiation will be black.
This is because the colour black is what is seen when all colours from the visible light spectrum are absorbed.
Temperature effects on radiation
All bodies (objects) emit a spectrum of thermal radiation in the form of electromagnetic waves.
As temperature increases, the object emits more radiation and the peak wavelength decreases, meaning the radiation shifts from infrared towards visible light as wavelength of radiation gets shorter.
Black body spectrum for objects of different temperatures
From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays).
At room temperature objects emit thermal radiation in the infrared region of the spectrum.
At around 1000 °C an object will emit a significant amount of red light.
At 6000 °C an object will mainly emit white or blue light (and some ultraviolet).
At even higher temperatures objects will emit ultraviolet or even X-rays.
Absorption and emission of radiation
As an object absorbs thermal radiation it will become hotter.
As it gets hotter it will also emit more thermal radiation.
The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation.
At this point, the object will be in equilibrium.
If, however, the object starts to absorb radiation again at a higher rate than it radiates it, then the object will heat up again.
Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down again.
The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected).
Temperature of the earth
The Earth receives the majority of its heat in the form of thermal radiation from the Sun.
At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (the surface temperature of the Earth is significantly smaller than the surface temperature of the Sun).
Some gases in the atmosphere, such as water vapour, methane, and carbon dioxide (greenhouse gases) absorb and reflect back longer-wavelength infrared radiation from the Earth and prevent it from escaping into space.
This process makes the Earth warmer than it would be if these gases were not in its atmosphere.
The greenhouse effect
The rate of absorption and emission of radiation on Earth contributes to the Greenhouse Effect.
This is the natural process that warms the Earth’s surface from the Sun.
The Sun’s thermal radiation reaches the Earth’s atmosphere where:
Some radiation is reflected back to space.
Any radiation not reflected is absorbed and re-radiated by greenhouse gases.
The absorbed radiation then warms the atmosphere and the surface of the Earth.
