|
Problem On page 217: What is the minimum frequency of incident radiation required to expel electrons from a copper surface? From a silver foil?
Subject Photoelectric effect, Fine: pages 170-174
Solution We can find the general equation for the photoelectric effect on page 173. This equation gives us the kinetic energy KE of electrons expelled from a surface by an incident radiation of a frequency n, if the workfunction of the metal, W, is known. Here, the workfunction for the metal can be found from table 5.2 on page 173.
In this special case, instead of a kinetic energy, we need to know a frequency of the incident radiation. The minimum frequency we need is found is we use a kinetic energy of zero, so that we get the special equation at the top of page 174:
| h n = W or, equivalent to obtain a frequency, n = |
W
h
|
, |
|
Filling in the workfunction W as found in table 5.2 (page 173), and the Planck constant h = 6.626 × 10-34 J· s (see page 168 or 171) we get, for the copper (Cu) surface a minimum frequency of:
| nCu = |
WCu
h
|
= |
7.18 ×10-19 J
6.626 ×10-34 J·s
|
|
|
and for the silver (Ag) foil a minimum frequency of:
| nAg = |
WAg
h
|
= |
7.37 ×10-19 J
6.626 ×10-34 J·s
|
|
|
(These frequencies both have a wavelength of the order of 300 nm (or 3000 Å), see table 5.2. Table 5.1 on page 196 shows that this is ultraviolet light.)
|
|
