question_answer

When monochromatic radiation of wavelength \[\mathbf{2000}\text{ }\overset{\circ }{\mathop{\mathbf{A}}}\,\] falls upon a nickel plate, the latter acquires a positive charge. The wavelength is increased and at\[\mathbf{3400}\text{ }\overset{\circ }{\mathop{\mathbf{A}}}\,\], however intense the monochromatic radiation may be, effect is found to cease. Give reason.

Answer:

                Clearly, \[{{\lambda }_{0}}=3400\overset{\circ }{\mathop{\text{A}}}\,\] is the threshold wavelength for nickel plate. When \[\lambda \le {{\lambda }_{0}},\]the incident photons have energy more than or equal to work function of Ni, so photoelectric emission occurs. When\[\lambda >{{\lambda }_{0}}\] energy of photons is less than the work function, so photo electric emission does not occur.