VISUAL  PHYSICS  ONLINE

7.1   THE NATURE OF LIGHT

        ELECTROMAGNEITC WAVES

 

P71 001

An experiment was performed using a photocell. The surface was illuminated by light of different frequencies and the stopping voltage was measured.

 

Define each of the terms: stopping voltage, threshold frequency, threshold wavelength, work function of the surface.

 

(A)     What is Einstein’s equation of conservation of energy explaining the photoelectric effect in terms of the stopping voltage and frequency?

 

wavelength (nm)

360

400

440

490

550

580

stopping voltage (V)

1.45

1.12

0.95

0.60

0.40

0.25

 

(B)     Plot the stopping voltage versus the frequency of the light.

 

(C)     Determine: the threshold frequency, threshold wavelength, the work function in eV and J, and the value of Planck’s constant.

 

(D)     Another surface was used in the experiment. Its work function was 0.5 eV. Draw a line on the graph for this surface.

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution

Stopping voltage VS: voltage required to reduce photocurrent to zero.  Maximum kinetic energy of emitted electron  Kmax =  e VS i.

Threshold frequency fC: Minimum frequency for the release of electrons from the surface of the material.

Threshold wavelength :  Maximum wavelength of an emitted electron form the surface of the material.

Work function Wmin: Minimum energy required to remove an electron from the surface.

(A)

Einstein used the principle of conservation of energy

              

 

(B to D)

slope                             m = 3.75´10-15  (V.Hz-1)

intercept                       b = -1.67 V

Work function              W = 1.7 eV = 2.7´10-19 J

Planck’s constant         h = 6.0´10-34 J.s      note – answer lower than accepted value

Threshold frequency    fc = 4.5´1014 Hz

Threshold wavelength          lc = 6.7´10-7 m = 670 nm

 

The two lines have the same slope = h / e.