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.
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.
