VISUAL  PHYSICS  ONLINE

7.1   THE NATURE OF LIGHT

        ELECTROMAGNEITC WAVES

 

P71 006

A student conducts an experiment using a photoelectric cell. Light is shone through a grid onto a metal surface. The metal is at earth potential and the grid is at 100 V, so that any electrons emitted from the surface produce a current in the external circuit. The student shines light sources of different photon energies onto the metal surface and records the current flowing for each. The light sources are adjusted so that their intensities are equal. The results are recorded in the table.

Photon energy (eV)     Photo-current (μA)   

                 0.5                                0

                 0.9                                0

                 1.2                                0.5

                 1.70                              2.8

                 1.75                              4.0

                 1.90                              8.0

                 2.20                              9.2

                 2.50                              9.4

(A)  Plot the graph (X-axis – photon energy).

(B)  Draw the straight line of best fit in the region where the photo-current varies greatest with photon energy.

(C)  From the line drawn on your graph, estimate the minimum energy (work function) for photoelectric emission. What wavelength for the incident em radiation does this correspond to and what part of the em spectrum?

(D) The experiment is repeated, but the intensities of the light sources are doubled. Predict the results of this new experiment by drawing a second line on the graph.

(E)  Explain the shape of the curves.

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

View solution below only after you have completed the answering the question.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution

(D)     Work function W = 1.6 eV

          lc = ?  m     W = 1.6 eV = (1.6) qe  J     qe = 1.602´10-19 C      c = 3.0´ m.s-1 

           W = h fc = h c /lc  = 2.563´10-19  C

          l = h c / W = 7.8´10-7 m = 780 nm     infrared

(E)     For photon energy less than the threshold frequency, no electrons are released from the surface irrespective of the intensity. For energies above the threshold, as the energy is increased the number of electrons released increases as shown by the increase in the current – the greater the intensity more photons – more electrons released à greater current. When the energy becomes sufficiently high, almost all the available electrons are released from the surface and there is only a very small increase in current for increased energy of the photons– saturation effect.