Black Body Radiation Video Lecture | Heat Transfer - Mechanical Engineering

welcome to electron line and to get a
better understanding of heat radiation
or energy radiation let's take a look at
something called the blackbody radiator
and so the best way to illustrate that
is to take a box completely enclose the
blocks like we have over here paint the
walls on the inside of the box black and
then make it tiny little hole so small
that no light or virtually no light will
actually enter the box and there will be
some radiation coming out of the box
which depends upon the temperature the
walls of the inside of the box and of
course that radiation will have a
specific wavelength and frequency
depending upon its temperature and from
Wiens law we know that the temperature
is equal to zero point zero zero to nine
divided by the wavelength and conversely
we could say that the wavelength of it
coming out of the box so we have certain
amount of radiation of particular
wavelength come out of the box will be
equal to zero point zero zero to nine
divided by the temperature in Kelvin of
course and but the the misunderstanding
may come from thinking that that will be
an exact wavelength and therefore will
give you radiation of an exact frequency
of course understanding that C is equal
to the frequency times the wavelength so
therefore that the frequency is equal to
the speed of light divided by the
wavelength so we take this wavelength
which depends on the temperature of the
inside of the box which then will give
us a corresponding frequency of the
radiation but it turns out it's not that
exact when we study the radiation coming
out of a box like that we notice that
it's not a single frequency or a single
wavelength that comes out but a
distribution of wavelengths and here I
have drawn a curve that represents a
distribution of the wavelengths coming
out of that box and this is known as the
black body radiation curve notice that
some wavelengths are shorter and some
length wavelengths are far longer than
the ones I would have gotten from using
this equation right here however the
wavelength the most predominant
wavelength of the radiation does
correspond to the temperature so we can
say that this wavelength right here that
gives us the greatest amount of
radiation coming from the box will
definitely depend upon the particular
temperature of the box but you do get
radiations
other types as well so that's what we
mean by the black body radiation curve
which means that when this was of course
the wavelength of the Sun and we have
the temperature the Sun that's now known
to be 5,800 degrees Kelvin on the
surface we then also see that the Sun
produces radiation of shorter
wavelengths that includes wavelengths of
ultraviolet radiation and longer
wavelengths which includes a radiation
of infrared but the predominant would be
visible light and of course the most
common visible light wavelength we get
from the Sun would be yellow light and
that's why the Sun looks yellow to us
all right so that gives you kind of an
idea I didn't want to confuse you or
didn't long want to give you the false
impression that the radiation coming
from an object is just a specific
frequency or a specific wavelength so
now in the next videos we are going to
go ahead and calculate the amount of
heat or the amount of energy radiated
out by different kinds of objects in
different kind of circumstances so let's
go to the next videos and see how that's
done