22.1 The Photoelectric Effect

State what is meant by a photon.

Electromagnetic radiation of a varying frequency f and constant intensity I is used to illuminate a metal surface. At certain frequencies, electrons are emitted from the surface of the metal.  

(ii)

The variation with f of the maximum kinetic energy E_MAX of the emitted electrons is shown in Fig. 1.1.

Fig. 1.1

 

Describe four conclusions that can be drawn from the graph in Fig. 1.1. The conclusions may be qualitative or quantitative.                                                   

                                              [1]      

The experiment in (b) is carried out twice more. Each time one change is made to the method used. 

State, and explain, how the graph obtained would compare with Fig. 1.1. when: 

(i)

the same metal is used, but with electromagnetic radiation of intensity 

 

[2]

State what is meant by the work function energy of a metal.

Ultraviolet radiation of frequency 8.5 × 10¹⁴ Hz is incident, in a vacuum, on a metal surface. The power of the radiation incident on the surface is 9.45 mW. Photoelectrons are emitted with a maximum kinetic energy of 2.1 × 10^–19 J. 
 Determine the number of photons incident on the surface per unit time.

Calculate the work function energy Φ of the metal.

The frequency of the radiation incident on the surface in (b) is decreased while the power remains constant 

State and explain the effect of this change on: