The Photoelectric E ffect
By
Eleanor Girdziusz
The Photoelectric Effect
“The phenomenon that when light shines on a
metal surface, electrons are emitted”
Wave Theory
Supported by such observations as:
• Refraction of light
• Reflection of light
• Interference patterns
• Diffraction of light
Wave Theory
• Energy from radiation spread equally
• Low intensity means no emission of
electrons
• Energies of the electrons do not
increase with intensity of the radiation
Quanta
In 1901, Max Planck suggested light was
made up of ‘packets’ of energy:
E = hf
h = Planck’s constant = 6.626x10-34Js
f = frequency of radiation
Photons
• In 1905, Albert Einstein presented the idea
that energy is not just emitted as quanta, but
continues to exist in this form
• Thus light is a stream of photons, each with
energy hf
Photoemission
• The number of electrons emitted by a surface is
proportional to the number of incident photons
• An electron is emitted as soon as a photon reaches
the surface
hf = φ + ½ mV2
φ = work function
½ mV2 = max kinetic energy
Threshold Frequency
hf < φ
hfo = φ
hc = φ
λo
c = speed of light
λo =
wavelength
Work Function
The minimum amount of energy that has to
be given to an electron to release it from the
surface of the material and varies depending
on the material
Gold Leaf Electroscope
Demonstration
The Photoelectric Effect
• UV light causes electrons to be released
from the surface of the zinc
• Emission of electrons commences at the
instant the surface is irradiated
• The frequency of the radiation must exceed
the threshold frequency
• Intensity of radiation has no effect