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Chapter 6

Radiative TransferRadiative Transfer

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Radiative TransferRadiative Transfer

The study of the propagation of energy by radiative processes; it is also called radiation transport. Radiation is one of the three mechanisms by which energy moves from one place to another, the other two being conduction and convection. See Electromagnetic radiation, Heat transfer

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A black body A black body is a theoretical object that absorbs 100% of the radiation that hits it. Therefore it reflects no radiation and appears perfectly black.

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This graph shows how the black body radiation curves change at various temperatures. These all have their peak wavelengths in the infra-red part of the spectrum as they are at a lower temperature than the previous graph. The graph shows:

As the temperature increases, the peak

wavelength emitted by the black body decreases.It therefore begins to move from the infra-red towards the visible part of the spectrum. Again, none of the graphs touch the x-axis so they emit at every wavelength. This means that some visible radiation is emitted even at these lower temperatures and at any temperature above absolute zero, a black body will emit some visible light. The graph also shows: .

As temperature increases, the total energy emitted increases, because the total area under the curve increases.

It also shows that the relationship is not linear as the area does not increase in even steps. The rate of increase of area and therefore energy increases as temperature

This graph shows how the black body radiation curves change at various temperatures. These all have their peak wavelengths in the infra-red part of the spectrum as they are at a lower temperature than the previous graph. The graph shows:

As the temperature increases, the peak

wavelength emitted by the black body decreases.It therefore begins to move from the infra-red towards the visible part of the spectrum. Again, none of the graphs touch the x-axis so they emit at every wavelength. This means that some visible radiation is emitted even at these lower temperatures and at any temperature above absolute zero, a black body will emit some visible light. The graph also shows: .

As temperature increases, the total energy emitted increases, because the total area under the curve increases.

It also shows that the relationship is not linear as the area does not increase in even steps. The rate of increase of area and therefore energy increases as temperature

Black body radiation curves showing peak Black body radiation curves showing peak wavelengths at various temperatureswavelengths at various temperatures

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Simple Macroscopic Model for the Interaction of Radiation with MatterSimple Macroscopic Model for the Interaction of Radiation with Matter

0ds

dIV

where s is the coordinate along the ray between the source at s=0 and the detector at s=so. What happens when there is an intervening medium between Sin and Sout?

Absorption between a source and a detector. The coordinate along the ray increases from Sin at the input end of the absorber to Sout at the output end of the absorber. The optical depth is measured in the opposite direction, starting from T=0 at Sout and increasing as s decreases.

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Think of the ray as a beam of photons, or light particles, some of which may be absorbed by the medium and vanish. The infinitesimal probability dp v of a photon with frequency v being absorbed (e.g., by hitting an absorbing particle) in a thin slab of thickness ds is directly proportional to ds: dpv=Kvds. This frequency-dependent constant of proportionality

AbsorptionAbsorption

dpv

dsv

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EmissionEmission

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Emission and absorption of radio waves in the Earth's atmosphere

Emission and absorption of radio waves in the Earth's atmosphere

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global warmingGlobal Warming is the increase of Earth's average surface

temperature due to effect of greenhouse gases, such as carbon

dioxide emissions from burning fossil fuels or from

deforestation, which trap heat that would otherwise escape

from Earth. This is a type of greenhouse effect.

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What is the greenhouse effect?

What is a greenhouse?

A greenhouse is a house made of glass. It has glass walls and a glass roof. People

grow tomatoes and flowers and other plants in them. A greenhouse stays warm

inside, even during winter. Sunlight shines in and warms the plants and air inside.

But the heat is trapped by the glass and can't escape. So during the daylight

hours, it gets warmer and warmer inside a greenhouse, and stays pretty warm at

night too.

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How is Earth like a greenhouse? Earth's atmosphere does the same thing as the greenhouse. Gases in the atmosphere such as carbon dioxide do what the roof of a greenhouse does. During the day, the Sun shines through the atmosphere. Earth's surface warms up in the sunlight. At night, Earth's surface cools, releasing the heat back into the air. But some of the heat is trapped by the greenhouse gases in the atmosphere. That's what keeps our Earth a warm and cozy 59 degrees Fahrenheit, on average.The gases on Earth act just like the glass; this is how the Earth gets warm from the sun even though it is about 93 million miles away. The gases allow the sun's rays to shine in, but then prevent the heat from escaping the Earth. This way of warming the Earth's surface is referred to as the greenhouse effect.

Greenhouse EffectHere are the main gases in the Earth's atmosphere that cause the greenhouse effect:Water vapourCarbon dioxideMethaneNitrous oxide

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