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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 330 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 331 332 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 333 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. 334 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. 335 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. 336 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. 337 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 338