NCEA Level 3 Physics Problems - Wave Particle Duality
Photoelectric Effect
Acceleration due to gravity,g = 9.81 Nkg-1
Speed of light = 3.0 x 108ms-1
Planck's constant = 6.6 x 10-34Js
Mass of electron = 9.1 x 10-31kg
Electronic charge = 1.6 x 10-19C
(1) When light is incident in a metal plate electrons are emitted only when the frequency of the light exceeds a certain
value. How has this been explained?
The maximum kinetic energy of the electrons emitted from a metallic surface is 1.6 x 10 -19J when the frequency of
the incident radiation is 7.5 x 1014 Hz. Calculate the minimum frequency for which electrons will be emitted.
(2) What do you understand by the quantum theory? Describe the evidence provided by experiments on the
photoelectric effect in favour of this theory.
Calculate the energy, in joules, of the photons associated with light of wavelength 3.0 x 10 -7m.
(3) When light of wavelength 5.0 x 10-7m falls on a surface it ejects photoelectrons with a maximum velocity of
6.0 x 105ms-1.
Calculate; (a) the work function in electron-volts
(b) the threshold frequency for the surface.
(4) (a) If a surface has a work function of 3.0 eV, find the longest wavelength light which will cause the emission of
photoelectrons from it.
(b) What is the maximum velocity of photoelectrons liberated from a surface having a work function of 4.0 eV by
ultra-violet radiation of wavelength 2.0 x 10-7m.
(5) In an experiment on the photoelectric effect using radiation of wavelength 4.00 x 10-7m the maximum electron
energy observed was 1.40 x 10-19 J. With a wavelength 3.00 x 10-7m the maximum energy was 3.06 x 10-19J.
Derive a value for Planck's constant. Give one other physical phenomenon involving Planck's constant.
(6) Write down Einstein's equation for photoelectric emission. Explain briefly the meaning of the terms in the equation
and discuss its significance.
Describe briefly how Einstein's equation may be verified experimentally.
An effective point source emits monochromatic light of wavelength 4500 Angstroms at a rate of 0.11 watt. How
many photons leave the source per second?
Light from the source is emitted uniformly in all directions and falls normally on the cathode of area 3.14 cm2 of a
photocell at a distance of 50 cm from the source. Calculate the photoelectric current, assuming 10 % of the photons
incident on the cathode liberate electrons. (1 Angstrom = 10 -10m)
(7) Describe and explain one experiment in which light exhibits a wave-like character and one experiment that
illustrates the existence of photons.
Light of frequency 5.0 x 1014Hz liberates electrons with energy 2.31 x 10-19J from a certain metallic surface. What
is the wavelength of ultra-violet light which liberates electrons of energy 8.93 x 10 -19J from the same surface?
(8) For caesium the work function is 1.35 eV.
(a) what is the longest wavelength that can cause photoelectric emission from a caesium surface?
(b) What is the minimum velocity with which photoelectrons will be emitted from a caesium surface illuminated
with light of wavelength 4000 Angstroms?
(c) What potential difference will just prevent a current passing through a caesium photocell illuminated with light
of wavelength 4000 Angstroms? (1 Angstrom = 10 -10m)
Atomic and Nuclear Physics
(9) When the light received from a star is examined, each line in the spectrum of a particular element in the star is seen
at a wavelength different from that of the same element when viewed in the laboratory. Most stars and