Thermal radiation: Characteristics
An object at any temperature emits radiation sometimes referred to as thermal
radiation. Characteristic of this radiation depends on the temperature and the
properties of the object.
At low temperatures, the wavelengths of thermal radiation are in the infrared
region, as the temperature increases the objects begins to glow red means enough
visible radiation is emitted so that object appears to glow.
As temperature increases, the thermal radiation it emits consists of a continuous
distribution of wavelengths from the infrared, visible and ultraviolet portions of the
spectrum.
From classical view point thermal radiation from accelerated charged particles in
the atoms near the surface of the object. The thermally agitated particles can have
a distribution of accelerations which accounts for the continuous spectrum of
radiation. This classical theory was inadequate.
The basic problem was in understanding the observed distribution of
wavelengths in the radiation emitted by a blackbody.
As the temperature increases, distinct behavior are observed. Peak of the
distribution shifts to shorter wavelengths.
Even 2022 PH 212 (Modern Physics) 1
, Application of heat radiation
1. White clothes are preferred in summer and dark colored in
winter.
2. Cooking utensils are blackened at the bottom and polished on
the upper surface.
3. Hot water pipes and radiators used in rooms are painted black so
that they can radiate maximum amount of heat to the room. The
same pipes outside the rooms are painted white so that they do
not lose heat to the surroundings.
4. Polished reflectors are used in electric heaters to reflect
maximum heat in the room.
Even 2022 PH 212 (Modern Physics) 2
,Blackbody (Need not appear to be black)
A material is constantly exchanging heat with its
surrounding (to remain at a constant temperature):
It absorbs and emits radiations.
Problem: it can reflect incoming radiations, which
makes a theoretical description more difficult
(depends on the environment) Gustav Kirchhoff
Blackbody (A perfect emitter and a perfect absorber of radiation)
Incoming radiations is totally absorbed regardless of the
frequency and none is reflected.
Sun and Earth’s surfaces behave approximately as
blackbodies.
The light emitted by a black body is called black-body radiation.
https://phet.colorado.edu/sims/html/blackbody-spectrum/latest/blackbody-spectrum_en.html
Even 2022 PH 212 (Modern Physics) 3
, Black body
A black body is an ideal body which allows the whole of the incident radiation to
pass into itself (without reflecting the energy ) and absorbs within itself this whole
incident radiation (without passing on the energy). This propety is valid for radiation
corresponding to all wavelengths and to all angels of incidence. Therefore, the black
body is an ideal absorber of incident radaition.
Black body radiation
curves showing peak
wavelengths at various
temperatures.
Even 2022 PH 212 (Modern Physics) 4
An object at any temperature emits radiation sometimes referred to as thermal
radiation. Characteristic of this radiation depends on the temperature and the
properties of the object.
At low temperatures, the wavelengths of thermal radiation are in the infrared
region, as the temperature increases the objects begins to glow red means enough
visible radiation is emitted so that object appears to glow.
As temperature increases, the thermal radiation it emits consists of a continuous
distribution of wavelengths from the infrared, visible and ultraviolet portions of the
spectrum.
From classical view point thermal radiation from accelerated charged particles in
the atoms near the surface of the object. The thermally agitated particles can have
a distribution of accelerations which accounts for the continuous spectrum of
radiation. This classical theory was inadequate.
The basic problem was in understanding the observed distribution of
wavelengths in the radiation emitted by a blackbody.
As the temperature increases, distinct behavior are observed. Peak of the
distribution shifts to shorter wavelengths.
Even 2022 PH 212 (Modern Physics) 1
, Application of heat radiation
1. White clothes are preferred in summer and dark colored in
winter.
2. Cooking utensils are blackened at the bottom and polished on
the upper surface.
3. Hot water pipes and radiators used in rooms are painted black so
that they can radiate maximum amount of heat to the room. The
same pipes outside the rooms are painted white so that they do
not lose heat to the surroundings.
4. Polished reflectors are used in electric heaters to reflect
maximum heat in the room.
Even 2022 PH 212 (Modern Physics) 2
,Blackbody (Need not appear to be black)
A material is constantly exchanging heat with its
surrounding (to remain at a constant temperature):
It absorbs and emits radiations.
Problem: it can reflect incoming radiations, which
makes a theoretical description more difficult
(depends on the environment) Gustav Kirchhoff
Blackbody (A perfect emitter and a perfect absorber of radiation)
Incoming radiations is totally absorbed regardless of the
frequency and none is reflected.
Sun and Earth’s surfaces behave approximately as
blackbodies.
The light emitted by a black body is called black-body radiation.
https://phet.colorado.edu/sims/html/blackbody-spectrum/latest/blackbody-spectrum_en.html
Even 2022 PH 212 (Modern Physics) 3
, Black body
A black body is an ideal body which allows the whole of the incident radiation to
pass into itself (without reflecting the energy ) and absorbs within itself this whole
incident radiation (without passing on the energy). This propety is valid for radiation
corresponding to all wavelengths and to all angels of incidence. Therefore, the black
body is an ideal absorber of incident radaition.
Black body radiation
curves showing peak
wavelengths at various
temperatures.
Even 2022 PH 212 (Modern Physics) 4
