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7.1   THE NATURE OF LIGHT

        QUANTUM MODEL

 

P71 012

When electromagnetic radiation shines on metals, photoelectrons may be emitted. The maximum kinetic energy of emitted photoelectrons is plotted against radiation frequency for four metals. Electromagnetic radiation of wavelength 187 nm shines upon an unknown metal and the maximum kinetic energy of the photoelectrons is found to be 2.5 eV. What is the unknown metal?

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution

Incident radiation

          l = 187 nm = 187´10-9 m                  c = l f         c = 3.00´108 m.s-1

          f = c / l  = 1.60´1015 Hz  

Max KE

          EKmax = 2.5 eV = (1.602´10-9)(2.5) J = 4.00´10-19 J

Photoelectric Effect

          h f = EKmax + W              W = h fc

Need to find threshold frequency fc

          f c = f – EKmax / h =  1.0´1015 Hz              à     Al