Radiation Pressure

Radiation Pressure

Electromagnetic waves have linear momentum as well as energy: This means that we can exert a pressure-a radiation pressure-on an object by shining light on it. However, the pressure must be very small because, for example, you do not feel  a camera flash when it is used to take your photograph.To find an expression for the pressure, let us shine a beam of electromagnetic  radiation-light, for example-on an object for a time interval t:.t. Further, let us assume that the object is free to move and that the radiation is entirely absorbed (taken up) by the object. This means that during the interval t:.t, the object gains an  energy t:.U from the radiation. Maxwell showed that the object also gains linear momentum. The magnitude t:.p of the momentum change of the object is related to the energy change U by where c is the speed of light. The direction of the momentum change of the object is the direction of the incident (incoming) beam that the object absorbs.  Instead of being absorbed, the radiation can be reflected by the object; that is,the radiation can be sent off in a new direction as if it bounced off the object. If the radiation is entirely reflected back along its original path, the magnitude of the momentum change of the object is twice that given above,

In the same way, an object undergoes twice as much momentum change when a perfectly elastic tennis ball is bounced from it as when it is struck by a perfectly inelastic ball (a lump of wet puny, say) of the same mass and velocity. If the incident radiation is partly absorbed and partly reflected, the momentum change of the object
is between t:.U/c and 2 t :.U/c. From Newton’s second law, we know that a change in momentum is related to
a force by

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