In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals P , Q and R are E P ,   E Q and E R , respectively, and they are related by E P = 2 E Q = 2 E R . In this experiment, the same source of monochromatic light

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In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals  P , Q  and  R  are  E P , &#160; E Q  and  E R , respectively, and they are related by  E P = 2 E Q = 2 E R . In this experiment, the same source of monochromatic light is used for metals  P  and  Q  while a different source of monochromatic light is used for the metal  R . The work functions for metals  P , Q  and  R  are  4 . 0 &#160; eV , &#160; 4 . 5 &#160; eV  and  5 . 5 &#160; eV , respectively. The energy of the incident photon used for metal  R , in  e V  is ___.

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Correct answer: A. A

Let the maximum kinetic energies of photoelectrons be E P = 2 E Q = 2 E R = 2 E . The energy of incident photon on metal  P &#160; and &#160; Q  be  U  and for  R  be  U ' . Photoelectric equation,  E ph = &#981; + K E max Now, using it for the three cases,  U = 4 + 2 E &#160; &#160; &#160; . . . 1 U = 4 . 5 + E &#160; &#160; &#160; . . . 2 U ' = 5 . 5 + E &#160; &#160; &#160; . . . 3 From  1 &#160; and &#160; 2 ,  E = 0 . 5 &#160; eV From  3 , energy of the incident photon for metal  R , U ' = 5 . 5 + 0 . 5 = 6 &#160; eV

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