Facts about thermal
radiations
1. Most of the solar radiations reaching
the earth are in the thermal radiation ranges.
2. Thermal radiations are emitted as a
result of thermal agitations
3. Thermal agitations are as a result of
increase in temperature.
4. Heat and light spectra of
electromagnetic radiations falls under thermal radiations.
5. Electromagnetic radiations with
wavelength between 0.2 to 10000 micrometer are part of thermal radiations
6. Thermal radiations are the visible
light, ultra violet and the infrared portions.
7. Thermal radiations can be divided
into short wave and the long wave.
8. The visible light is part of thermal
radiations and falls with wavelength between 0.39 to 0.77micrometer.
9. The intensity of solar radiation
impinging on a surface depends on angle of incident, time of the year and atmospheric
condition.
10.
The
infrared region is divided into near infra red and far infra red.
11.
Emission
of radiations is due to oscillations and transitions of electrons that comprise
matter and is sustained by the thermal energy of the matter.
12.
Emission
correspond to heat transfer from the matter and hence a reduction in thermal
energy stored by the matter.
13.
Radiation
may also be intercepted absorbed by matter.
14.
Absorption
of radiation result to heat transfer by the matter and hence an increase in
energy stored by the matter.
15.
Emission
from gas or transparent liquid or solid is a volumetric phenomenon.
16.
Emission
from an opaque solid or liquid is a surface phenomenon.
17.
For
an opaque object emission from atoms and molecules within one micrometer depth to
the surface.
18.
All
bodies emit radiations at temperature above 0k, absolute zero.
19.
A
transparent body will transmit all radiations that falls on it.
20.
A
translucent body will scatter radiation falling on it.
21.
An opaque object will absorb or reflect
radiation falling on it.
22.
Scatting
is the diffused reflection of light in a body.
23.
The
absorption and reflection of radiations depend on the color and size of the
molecules of the object involved.
24.
When
a body absorb radiations it temperature rises and it start to emit radiations.
25.
It
is only a black body that can absorb or reflect all wavelength of light at
temperature and all angles of incidence.
26.
When
wavelength of radiation is greater than size of atom or molecules a body the wave
will be scattered.
27.
Change
in temperature in matter cause charge oscillations which in turn produce
electromagnetic radiations.
28.
Sunlight
is part of electromagnetic radiations produced by the hot plasma of the sun.
29.
Absorptivity
, reflectivity and emissivity of all objects depend on wavelength of
radiations.
30.
The
frequency of distribution of thermal radiations is given by plank’s law.
31.
Increase
in the rate of thermal radiation is proportional to the 4th root of
its temperature T4.
32.
Objects
absorb much of the frequency range they can emit.
33.
The
rate at which objects radiates (or absorb) depends on the nature of the surface.
34.
Object
that is a good emitter is also a good absorber.
35.
The
hotter an object is the more light it emits.
36.
Stephan-Boltzmann
law states that the total energy radiated per unit surface area of a blackbody
across all wavelengths per unit time is proportional to the fourth power of its
absolute temperature.
37.
Emissivity
is the measure of the effectiveness of emission of thermal radiation by a surface.
38.
Absorptivity
of a surface is the measure of its effectiveness in absorption of radiations.
39.
The spectral distribution of radiation
intensity is explained by plank’s law.
40.
Reflectivity
is the ratio of the reflected to the incident radiation impinging a surface.
41.
Transmissivity
is the ratio of transmitted to incident
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