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Using the, "Radiation Pressure of Light"
to explain electromagnetic waves as an energy source.
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Radiation is sometimes a difficult topic to
explain because it falls under the category of invisible
principles, but in reality radiation can be seen in the form
of light and felt in the form of heat. Light and heat are
forms of energy. The power of light (a form of radiant
energy) can be demonstrated by converting it's
form into a form of kinetic energy as shown below. The key
message to understand is radiation can be seen and felt and
doesn't need air as an interstitial medium to reveal itself....and
unlike air which can simultaneously exists in the same space
at different densities and causes convection aka "cool
air falls / hot air
rises", radiant travels in all directions ergo it's
incorrect to say heat rises when clearly it can travel in
any direction - always from hot to cold.
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Figure 1. The radiation pressure of direct
sunlight is about 5 x
 N/ .
The effect of even less radiation can be shown with an
apparatus where disks are placed at the ends of bars that
are connected to a bearing. If one side of each disk is
black and the other side is a mirror, then the light
striking the black side is completely
absorbed, so all the
momentum of the light is transferred to the disk, while the
light incident on the mirror is almost all reflected.
Therefore the momentum transferred to the mirror is about
twice as great as that transferred to the disk facing the
light with its black side. The difference in momentum causes
the rotor to rotate. It is important to ensure low friction
at the bearing for the apparatus to work. In order to
eliminate the effects of thermal expansion and air friction,
the apparatus must be under high vacuum.
Citation: Sándor Kabai,
The Radiation Pressure of Light, Wolfram Demonstrations
Project, October 3, 2008
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