Download 33-6 Radiation Pressure

33-6 Radiation Pressure
Electromagnetic waves  have linear momentum as
well as energy  exerted a radiation pressure on an
object by  shining light on it.
the pressure  must be very small  like a camera
flash  every photographic flash could be like a
punch.
Finding an expression  for the pressure  by
shining a beam of electromagnetic radiation  light
 on an object for a time interval  t
Radiation absorbed  the object absorbs the
radiation  during the time t  will gains an energy
U
Maxwell showed  the object also gains linear
momentum 
The magnitude  p related to the energy change 
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C  the speed of light.
The momentum direction  is the direction of the
absorbed  or reflected beams
Radiation reflected  the object reflects the radiation
 the magnitude of the momentum change the object
 is twice than that in Eq.(33-28)
Radiation  is partly absorbed and partly reflected,
the momentum change of the object is between
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Ans: a) same ,
b) decrease
33-7 PoTarization
In England VHF  television antennas  oriented
vertically  the transmitting equipment  designed
to produce waves  their electric field oscillates
vertically.
In North America  television antennas  oriented
horizontally  the transmitting equipment 
designed to produce waves  their electric field
oscillates horizontally.
The difference  is due to the direction of oscillation
of the electromagnetic waves carrying the TV signal.
Figure 33-9a  shows an electromagnetic wave with
its electric field oscillating parallel to the  vertical y
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axis.
FIG.33-9 (a) The plane of oscillation of a polarized
electromagnetic wave. (b)To represent the polarization,
we view the plane of oscillation head-on and indicate
the directions of the oscillating electric field with a
double arrow.
The plane containing  E vectors is plane of
oscillation of the wave  plane-polarized in the y
direction
In Fig. 33-9b  indicates the wave's polarization 
as the wave travels past us  its electric field
oscillates vertically  continuously changes between
 directed up and down the y axis.
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Polarized Light
Television station  emitted electromagnetic waves
 have the same polarization,
Common sources  sun  bulb emitted light 
unpolarized electromagnetic waves  E at any given
point is always perpendicular to the direction of travel
of the waves  changes directions randomly.
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Unpolarized electromagnetic waves  couldn’t
represent easily  have a mess of double arrows
Fig. 33-10a  simplify the mess  by resolving each
electric field  into y  z components
As the wave  travels past us  the net y component
oscillates parallel to the y axis  z component
oscillates parallel to the z axis.
FIG.33-10b  represents  Unpolarized light  with
a pair of double arrows.
The double arrow  along the y axis represents the
oscillations of the  net y component of the electric
field
The double arrow  along the z axis represents the
oscillations of the net z component of the
electric field
Unpolarized light  change into the superposition of
two polarized waves whose planes of oscillation are
perpendicular to each other  one plane contains the
y axis and the other contains the z axis.
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Change make  to let drawing Fig. 33-10b  is a lot
easier than drawing Fig. 33-10a.
Fig. 33-11 shows  transform unpolarized visible
light into polarized light  by sending it through  a
polarizing sheet.
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Polarizing sheet  known as Polaroids or Polaroid
filters  invented in 1932 by Edwin Land  an
undergraduate student
Polarizing sheet  consists of certain long molecules
embedded in plastic  stretched to align the
molecules in parallel rows
When light sent  through the sheet  electric field
components along one direction pass through the
sheet  components perpendicular to that direction 
absorbed by the molecules and disappear
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