January 11th, 2013
• Video clip of the day.
• Quantum Theory
• Energy Levels
• The photoelectric effect confirms the theory of
the quantization of energy. Hertz was attempting
to verify Maxwell's theories. (≈ 1887) He
assembled a circuit that generated an oscillating
current that caused sparks to jump back and
forth across a gap.
• He showed that the sparks were generating
electromagnetic waves by seeing sparks forming
in the gap of the receiver.
• With this, he verified Maxwell's theories
that when the metal electrodes were
exposed to ultraviolet light, the sparks
• 10 years later, the electron was
discovered. Physicists suggested that the
UV light had ejected electrons from the
• The ejection of electrons by UV light is the
• Lenard was the first to perform detailed
experiments on the photoelectric effect.
– He designed an apparatus where electrodes
were sealed in a vacuum tube with a quartz
window. (UV light does not penetrate glass.)
• Lenard wanted to determine the kinetic
energy of the photoelectrons. When the
emitter was - and the collector was +, he
found that when the intensity increased,
the current increased.
• When he switched the polarity of the
emitter and the collector, he found that
when he increased the potential difference
(voltage), the current would eventually
Results from Testing:
• i) As the intensity of the light increases,
energy absorbed by the surface increases
and the number of photoelectrons
increases. (classical physics)
– more intense light more energy absorbed
by metal more electrons ejected from
• ii) Classical physics also indicates that as
the kinetic energy increases, the intensity
– However; Lenard found that the kinetic energy
is not affected by the intensity.
• Kinetic energy is determined ONLY by the
frequency of the light.
• KE dependent on frequency not
• Einstein proposed that 'photoelectrons' are
only ejected from the surface of the metal
terminal if the frequency of the EMR was
above a certain minimum level called the
threshold frequency, fo.
• Brightness or intensity of EMR had nothing to do
with it, as EMR wave theory had predicted.
• In 1905, Einstein explained the photoelectric
effect by saying EMR consisted of discrete
bundles of energy which he called photons and
that atoms would only absorb energy in these
• What Planck was essentially saying was that
energy emitted or absorbed by atoms is
QUANTIZED or exists in packages (bundles) of
energy in specific sizes.
BBC – Clash of the Titans
• 17 – 40 minutes
– History an experiments which were the origin
of Quantum Theory
Photo Electric Effect:
Photo Electric Effect:
Check out this website:
• High resolution screen needed….
• Photons have an energy, KE. They either
give up all their energy or none of it.
• The energy possessed by these photons
depended on the frequency of the EMR.
– E = hf
• Einstein was the first one to indicate that
light and other EMR acted like particles.
• Kinetic energy (Ek) of electron freed by
photoelectric effect is given by:
• Expressing extremely small amounts of
energy at the atomic level is usually done
in units called electron volts (eV).
• 1 eV = 1.6x10-19 J