VISUAL  PHYSICS  ONLINE

7.1   THE NATURE OF LIGHT

        ELECTROMAGNEITC WAVES

 

P71 002

 

(A)

What is the energy of a photon of wavelength 550 nm (joules and electron volts)?

What is the frequency and what part of the electromagnetic spectrum for this photon?

 

(B)

A student carried out an experiment during which light of different frequencies was

shone onto a metal surface to produce electrons. The student measured the maximum kinetic energy of the emitted photoelectrons as the frequency of light was altered.

(B1)   How did the student measure the maximum kinetic energy?

(B2)   What is the mathematical relationship between the maximum kinetic energy of the photoelectrons and the frequency of the light incident on the metal surface? Define each term in the equation and its SI unit.

(B3)   How could the student best analyse the data to determine a value for Planck’s constant?

 

(C)

Photoelectrons are emitted by a surface of a certain metal when the surface is illuminated by both violet light of wavelength 400 nm and green light of wavelength 550 nm but no photoelectrons are released from the surface by red light of wavelength 715 nm.

(C1)   Calculate the frequencies and the photon energies of the three light beams. 

(C2)   Explain the differences in energies of the electrons released by the violet and green light.

(C3)   Explain why no electrons are released when illuminated by red light.

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution

(A)

E = ? eV = ? J     f = ? Hz

l = 550 nm = 550´10-9 m

c = f l     E = h f = h c / l

f = c / l = 5.5´1014 Hz

E = 3.6´10-19 J = 2.3 eV

visible

 

(B)

Using a reverse voltage to reduce the photocurrent to zero à stopping voltage.

         

          

         

        

Plot Vs against f à slope = h/e à h

 

(C)

violet     l1 = 400 nm = 400´10-9 m     f1 = ? Hz     E1 = ? J = ? eV

green     l2 = 400 nm = 400´10-9 m     f2 = ? Hz     E2 = ? J = ? eV

red         l3 = 715 nm = 715´10-9 m     f3 = ? Hz     E3 = ? J = ? eV

f1 = 7.5´1014 Hz     E1 = 5.0´10-19 J = 3.1 eV

f2 = 5.5´1014 Hz     E2 = 3.6´10-19 J = 2.2 eV

f3 = 4.2´1014 Hz     E3 = 2.8´10-19 J = 1.7 eV

 

Violet light photons have 0.9 eV more energy à released electrons will have 0.9 eV extra kinetic energy.

 

The energy of the red photons is less than the work function of the surface à no electrons released from surface.