January 11th, 2012
• Video clip of the day.
• Collecting visiting the relatives
• Upcoming Assignments
• Planck’s Constant
• Wave Duality
• Atomic Theory
Wave - Particle Duality of Light
• Modern age physics accepts that light
sometimes acts as a wave and at other
times acts like a particle.
• Both matter and electromagnetic energy
exhibit some properties of waves and
some properties of particles.
• Small Wave Length:
– Behaves like a particle
• Large Wave Length:
– Behaves like a wave.
• Major discovery supporting Quantum Theory
• Bohr model, emission spectra, energy
– only 4 visible lines appeared when hydrogen
atoms were energized.
• PHYSICS 123 Demonstration: 9.1
• How do we explain this as a model for an
• The Problem:
– Electrons moving around a nucleus in roughly
circular or elliptical paths would feel a
centripetal force and would therefore have
– But accelerating charges produce electric
magnetic radiation, therefore these electrons
should continuously emit energy, slow down,
and eventually collapse into the nucleus.
– Obviously, this does not happen.
• Laws of electromagnetism do not apply at
• Within an atom energy is only absorbed or
emitted when electrons change energy levels
and this energy is QUANTIZED.
• electrons are either in lowest energy state
(ground state) or certain allowed levels (excited
• To move from one level to another,
electrons must emit or absorb a photon
with a certain amount of energy (E = hf).
• This absorbed or emitted photon had an
energy value equal to the difference
between energy levels.
• Bohr found that for the hydrogen atom, the
energy associated with a particular level was
• Where n is principle quantum number (energy
level) and energy is negative because energy is
being added to pull the electron away from the
Changing Energy Levels