Electromagnetic Radiation and
Energy
Wave Description of Light
• Electromagnetic radiation (ER): form of
energy that exhibits wavelike behavior as it
travels through space
• All forms of ER together make the
electromagnetic spectrum
• All forms of ER move at a constant speed of
about 3.0 x108 m/s (speed of light, c)
• Wavelength (λ): distance between
corresponding points on adjacent waves.
• Frequency (ν): number of waves that pass a
specific point in a given time, usually one
second.
• Unit: Hertz (Hz), aka (1/s) or (s-1)
• For electromagnetic radiation, frequency and
wavelength are related
C=λν
• If λ increases, what must happen to ν? Does c
change?
Practice
• Calculate the wavelength of a wave with a
frequency of 7.0 x 1016 Hz.
• Pp 20 #3,4
Photoelectric Effect
• Photoelectric effect: emission
of electrons from a metal
when light shines on that
metal
• Experiment: no e- were
emitted if light’s frequency
was below a certain threshold,
regardless of time shone.
• According to wave theory,
shining any light long enough
should supply enough energy
to eject an e-
Particle description of light
• Max Planck, 1900s, suggested that object emit
energy in small, specific amounts called
quanta
• Quantum: minimum amount of energy that
can be gained or lost by an atom
• Planck proposed a relationship between a
quantum of energy and the frequency of
radiation
E=hν
• E is energy, in Joules, of a quantum of
radiation
• h is Planck’s constant (fundamental physical
constant)= 6.626 x 10-34 J*s
• ν is freqency of radiation
• 1905 Einstein expands on this idea. ER have
dual wave/particle nature.
• While light emits many wavelike particles, it
can also be thought of as a stream of those
particles
• Einstein named the particles photons
• Photon: particle of ER having zero mass and
carrying a quantum of energy
• Energy of a particular photon depends on the
frequency of the radiation
Ephoton= hν
• Einstein’s Explanation: ER is absorbed by matter
only in whole numbers of photons
– For e- to be ejected, must be struck by single photon
possessing at least minumum energy
– According to equation, this energy corresponds to
frequency
– If photon’s frequency is below minimum, no e- ejected
Practice
• PP 20 Work problems 1-2 and 5-7
The Hydrogen Atom
• Ground state: lowest energy state of an
atom
• Excited state: atom has higher potential
energy than ground state
• When an excited atom returns to ground
state, it gives off energy it gained in the
form of ER.
• Production of colored signs (neon) is
example
Experiment
• Pass electrical current through H gas in
vacuum tube at low pressure
• Emits characteristic pink glow
• When the light was passed through prism,
separated into series of specific frequencies
and therefore wavelength (… equation?) of
visible light
• These bands are Hydrogen’s line emission
spectrum
Problem?
• Classical theory predicted that H atoms would
be excited by whatever amount of energy was
added to them.
• Expected to observe continuous range of
frequencies of ER, or continuous spectrum.
• Why had H only given off specific frequencies
of light?
•  Quantum Theory
• Excited H atom falls back from excited state to
ground state, and emits photon
• This energy is equal to difference between
initial and final state
Implications
• Since H atoms emit only specific frequencies,
difference between energy states must be
fixed.
• Therefore, e- of H atom exists only in very
specific energy states
• In 1913, Bohr proposed a model that linked
the atom’s electron with photon emission
• Energy is higher in orbits farther from
nucleus (like a ladder)
• Based on the wavelengths of hydrogen’s line
emission spectrum, Bohr calculated energies
the e- would have in the allowed energy level
for H atom.
• Bohr’s calculated values agreed with
experimentally observed values for lines in
each series
• Scientists tried to apply this model to other
element’s atoms