We all know that a light bulb emits visible light. However,
did you know that a light bulb also
produces invisible light that still falls under the category of radiation? When
one looks at a light bulb, or the sun, they observe a blinding white light, which
in reality is the superposition of the seven colors of the rainbow. In 1800, Sir William Herschel used Newton’s
research and discovery of the visible light spectrum to test for the influence
of light on temperature. In doing so, Herschel discovered that not only does
light carry warming energy, but it also emits radiation in both visible
spectrum forms and invisible infrared and ultraviolet forms. By determining
that light emits radiation scientists were able to further expand their
knowledge of energy and light and did so by analyzing wavelengths.
Thermal radiation can be defined as the electromagnetic energy
emitted by a surface. This radiation travels in all directions and continues to
travel until it reaches its point of absorption.
To fully grasp the concept of thermal radiation one must understand electromagnetic energy which is when
electric and magnetic fields, making up electromagnetic waves, flow through
space at the speed of light until they are absorbed when the energy of these
electromagnetic waves are transferred to the matter that they pass through. Thermal
radiation is measured in wavelengths and lives within the light spectrum
defined by Newton and evolved by Herschel.
Radiation
is measured in terms of wavelengths (See:
Let’s Shine A Little Light!). Colors are then defined by their corresponding
wavelength. For example infrared (invisible light at the exterior of the red
portion of the visible spectrum) has a wavelength of approximately 0.001mm, red
light has a wavelength of 0.0007mm, blue light has a wavelength of 0.0004 mm,
and ultraviolet light (invisible light at the exterior of the blue portion of
the visible spectrum) has a wavelength that is far too short to detect with the
naked eye. Based on these numbers and our knowledge of intensities linked to
color, one can note that the intensity of the radiation increases significantly
as the wavelengths shorten. These wavelengths correspond with the energy of photons and the energy level of
individual radiative objects impacts the color of these objects. Higher energy
concentrations per surface area result in higher temperatures, than lower energy
concentrations within the same surface area. Principally, regardless of the
surface area, hotter object appear bluer than cooler objects, which appear red.
After
having fully examined the facts regarding radiation and its link to light and
wavelengths, we turn our attention to
four principles of thermal radiation. 1)
The thermal radiation emitted by an object at any temperature consists of a
wide range of frequencies. This rule means that the frequency or intensity of
an object is distributed throughout the visible spectrum, which is referred to
in Planck’s Law of Black-Body Radiation.
2) The dominant frequency range of
the emitted radiation shifts to higher frequencies as the temperature of the
emitter increases. Wein’s Displacement Law
can explain this phenomenon and argues that the larger the kinetic energy the
shorter the peak wavelengths. This principal is based on the fact that all
atoms and molecules remain in a constant motion and that dense objects, or
blackbody objects, constantly emit thermal radiation. 3) The total amount of radiation of all frequencies increases as
the temperature rises. Stefan-Boltzmann’s
Law proves this principal and shows that the total radiative density of a
blackbody rises to the fourth power of the absolute temperature. This shows
that radiative density and the objects temperature are proportional to each
other. 4) The rate of
electromagnetic radiation emitted at a given frequency is proportional to the
amount of potential absorption of a source. This means that objects that emit
high frequencies also absorb high frequencies of energy and vise versa.
Blackbody sources, such as the sun, are objects that absorb all radiant energy and
also emit radiation at all wavelengths. Since blackbody objects can emit all
wavelengths they tend to emit the highest intensity radiative waves and thus
shine the brightest.
These aforementioned four
properties of thermal radiation help to describe influences of temperature,
energy, and size on radiation. Light radiation is measured by wavelengths in
terms of the light spectrum.
Sources:
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