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Absorption/emission of EM radiation: 2. Relationship between temperature and intensity, frequency and wavelength of radiation emitted from a surface - black body radiation

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EMR shorthand for electromagnetic radiation

INDEX of physics notes: Absorption & emission of EM radiation by materials - temperature and surface factors


2. The relationship between temperature and the intensity, frequency and wavelength of the electromagnetic radiation emitted from a surface

The power (P) per unit area is a measure of the intensity of radiation (units e.g. W/m2).

power units: joules/second (J/s) or Watts (W)

so here the intensity of EM radiation can be expressed as the rate at which energy is emitted per unit area.

Reminder - all objects are constantly emitting electromagnetic (EM) radiation over a range of frequencies depending on the temperature of the material.

The distribution and intensity of emitted wavelengths only depends on the temperature.

Black body radiation - absorption and emission

Absorption

An object that absorbs ALL radiation falling on it, at all wavelengths (or frequencies) , is called a black body - the 'perfect' or 'ideal' absorber of EM radiation.

However, most objects reflect light to some extent.

Graphite powder can absorb 97% of incoming radiation and I assume it can emit 97% of black body radiation?

There is military interest in blackbody-like materials for camouflage and radar-absorbent materials for radar invisibility - the idea is to avoid detection from reflected or emitted EM radiation.

Graphene nanostructure materials have been made with almost perfect black body properties.

Artists are interested in these graphene materials to produce the perfect black surface!

Emission

When a black body is at a specific uniform temperature, its emission has a characteristic wavelength (or frequency) distribution that depends ONLY on the temperature.

Its emission is called black-body radiation - the 'perfect' or 'ideal' emitter.

 

Looking at the distribution and intensity of emitted wavelengths/frequencies at different temperatures

 The intensity of emission for particular wavelengths/frequencies depends on the temperature of the object.

Intensity is power per unit area (e.g. units can be W/m2 or J/sm2)

Graph 1

The effect of temperature on the intensity - wavelength distribution is shown graph 'sketch' 1.

Whatever the temperature, the general shape of the graph is the same,

and ALL intensities increase in value for all wavelengths with increase in temperature.

From T1 to T4 represents a temperature range from ~1000 to 5000 K (~727oC to 4727oC)

Compared to a star surface, T1 is a relatively cool temperature e.g. glowing coals on a fire.

You would find that an object at room temperature has a curve lower than T1 and peaking more to the right.

T4 could represent the surface of a very hot star, the surface of our Sun is ~6000oC, so we get lots of visible light, and lots of ultraviolet radiation if it wasn't for the ozone layer above us!

As you go from T1 to T4 the object will shine more and more brightly - increase in overall intensity.

The wavelength with the highest intensity ('peak') of emitted radiation is called the principal wavelength.

When you heat an object from a low temperature to a high temperature you observe a sequence of colours.

e.g. when you heat a metal to a high temperature it changes from red, yellow, blue and then white.

The higher the temperature of an object the greater the intensity of every emitted wavelength,

AND, the higher the temperature the smaller the peak wavelength (or the higher the peak frequency).

Looking at graph 1 you can see that the intensity increases much more for shorter wavelengths (higher frequencies) than longer wavelengths with increase in temperature.

This is because shorter wavelength EM radiation transfers more energy.

The energy of EM waves is directly proportional to frequency

This results in the principal wavelength being decreased (gets shorter) the higher the temperature.

Therefore as objects get hotter the principal wavelength gets shorter and the intensity distribution gets wider and less symmetrical.

Graph 2

The effect of temperature on the intensity - frequency distribution is shown graph 'sketch' 2.

Note that ALL intensities increase in value for all frequencies with increase in temperature.

The frequency showing the greatest intensity ('peak') of emission is called the principal frequency.

The higher the temperature of an object the greater the intensity of every emitted frequency.

The principal frequency increases with increase in temperature of the object.

This means, as already stated, the principal wavelength of greatest emission intensity decreases with increase in temperature.

Astronomers use spectral data to identify elements in distant stars BUT can also use the wavelength/frequency distribution and intensity to work out the temperature of a star.

A hotter star will have a greater principal frequency (shorter wavelength) than a cooler star.

 

INDEX physics notes: Absorption and emission of EM radiation by materials


Keywords, phrases and learning objectives for absorption and emission of radiation

Appreciate the concept of black body radiation.

From graphs, understand the relationship between temperature, intensity and frequency (and wavelength) of electromagnetic radiation emitted or absorbed from/by a surface.

Know that as the temperature of surface increases, the frequencies of emitted radiation tend to increase (and the wavelength decreases).


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INDEX physics notes: Absorption and emission of EM radiation by materials

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