question_answer 27)

    An EM wave of intensity falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the followings are true?   (a) Radiation pressure is c if the wave is totally absorbed (b) Radiation pressure is /c if the wave is totally reflected (c) Radiation pressure is 2 /c if the wave is totally reflected (d) Radiation pressure is in the range/c < p < 2/c for real surface

Answer:

  (a,c,d)Radiation pressure (p) is the force exerted by em wave on unit area of the surface, i.e., rate of change of momentum per unit area of the surface. Momentum per unit time per unit area Change in momentum per unit time per unit area =  radiation pressure (p), i.e; p =  Momentum of incident wave per unit time per unit area = . When wave is fully absorbed by the surface, the momentum of the reflected wave per unit time per unit  area=0. Radiation pressure (p) = change in momentum per unit time per unit area = When wave is totally reflected, then momentum of the reflected wave per unit time per unit area = Radiation pressure (p) = Here , p lies between and . Thus options (a),(c) and (d) are correct