PHOTOELECTRIC
EFFECT
QUICK REVIEW
• DUAL NATURE OF LIGHT: Light is an
electromagnetic wave that has both wave
and particle properties.
• QUANTUM THEORY: Light consists of
bundles of energy called photons. The
energy is directly proportional to the
frequency of the light.
ENERGY OF A PHOTON:
E=hf
or
E=hc
λ
E = photon energy ( J or eV)
C =speed of light (3.0x108m/s)
h = Planck’s constant
(h= 6.63x10-34J·s)
f = frequency (Hz)
λ = wavelength (m)
EXAMPLE
Ex: A photon of red light has a frequency
of 4.22x1014Hz. How much energy does
it have in joules? In electron volts?
E = hf
E = 6.63x10-34J·s(4.22x1014Hz)
E = 2.80x10-19J= 2.80x10-19J
1.6x10-19J/eV
E= 1.75eV
Energy vs.
Frequency
Energy vs.
wavelength
E(J)
E(J)
f(Hz)
λ(m)
Important terms
• Photoelectric effect- The emission of an
electron from a metal plate due to
exposure to electromagnetic waves of a
certain frequency.
• Threshold Frequency (fo)- minimum
frequency needed to move photoelectrons.
Varies with each metal.
If the frequency is too low, no electrons are emitted.
If the frequency is equal to the threshold frequency,
the electron is emitted but doesn’t move.
If the frequency is greater than the threshold
frequency, the electron is emitted from the plate and
increases in velocity.
Kinetic energy of photoelectrons
Threshold frequency is
different for each metal.
As the frequency increases,
the kinetic energy of the
emitted electron increases.
The slope of every graph is
equal to Planck’s constant.
KE(J)
Fo (Hz)
NOTE: KE = 1/2mv2
If velocity increases so
does the kinetic energy
LIGHT INTENSITY
Increasing the intensity of
the light (brightness) will
increase the number of
released electrons but it
won’t increase the kinetic
energy of the electrons.
KE(J)
INTENSITY
Solar Cells:
Sunlight hits a metal
plate and releases
electrons. This
causes a flow of
electrons which
produces electrical
energy.
silicon, cadmium telluride, and copper indium
selenide-sulfide.