Chemistry
Ms. Pollock
2013 - 2014
6.1 The Dual Nature of Light
Introduction
◼ Understanding
of behavior of electrons in
electron cloud required revision of thinking
about matter and energy
◼ Mechanics study of motions of objects
under influence of forces
◼ Based on work of Isaac Newton in 1600s
◼ Quantum mechanics – study of behavior
of matter at atomic and subatomic level;
based on work of many scientists
Properties of Waves
◼ Different
experiments with light = different
indications about nature of light
◼ Energy waveforms (water/sound) exhibit
certain characteristics
▪ Diffraction – bending of waves around corners
▪ Interference – adding or subtracting of
energies when waves overlap
Properties of Waves
Properties of Waves
◼ Interference
▪ Overlapping waves
create new patterns
▪ Crests lining up or
troughs lining up –
amplitude increased;
constructive
interference
▪ Crests lining up with
troughs – amplitude
decreased; destructive
interference
Light as a Wave
◼ Light
able to undergo
diffraction and
interference
◼ Properties
demonstrated with
double slit
experiment
Light as a Wave
◼
Light behaved like
particles – slits to back
of box as two bright
spots
◼ Light behaved like
waves – slits to back of
box as bright spots
with areas of
constructive and
destructive interference
◼ Light behaving like a
wave
Light as a Particle
◼ Planck
studying
black body radiation
◼ Black body object
that absorbs all light
that falls on it –
reflects no radiation
and appears
perfectly black
◼ Black body radiation
– energy that would
be emitted by ideal
black body
Light as a Particle
◼
1900 Planck published
paper on
electromagnetic
radiation emitted from
heated black object
◼ Radiation emitted in
discrete bundles of
energy called quanta
◼ Quantum – small unit
into which certain
forms of energy are
divided
Light as a Particle
◼ Energy
of quanta
related to
frequencies
◼ Energy calculated
with Planck’s
constant
◼ E = hƒ
◼ Photon – particle of
light
◼ In discussion of light,
photon and quantum
are used
interchangably.
◼ What
is the frequency
of a photon of light
whose energy is 3.00
X 10-19 joules?
◼ ƒ = E = 3.00 X 10-19 J
=
h 6.63 X 10-34
J⋅s
E = 4.52 X 1014
Hz
Light as a Particle
◼
What is the energy of a
photon whose
frequency is 2.00 X
1015 s-1?
◼ E = hƒ
◼ E = (6.62 X 10-34 J⋅s) ⋅
(2.00 X 1015 s-1)
◼ E = 1.33 X 10-18 J
Planck’s work basis for
quantum theory
◼ Energy only able to
exist in discrete
amounts (quanta)
◼ Hard to measure since
molecules are so small
◼ Observable in bond
length expansion and
contraction – only
certain vibration levels
available to molecules
◼
Light as a Particle
effect –
phenomenon in
which electrons are
emitted from the
surface of a material
after the absorption
of energy
◼ Light strikes metal
surface with enough
energy to knock off
electrons
◼ Photoelectric
Light as a Particle
◼
Low frequency light
unable to knock off
electrons
◼ High frequency light
able to knock off many
electrons
◼ Need specific energy
to remove electrons,
since energy is
quantized
◼ Source of Einstein’s
Nobel Prize
Wave-Particle Duality
◼ Some
evidence light is wave
◼ Some evidence light is particle
◼ Led to development of wave-particle
duality of light
◼ Wave-particle duality – light travels as
wave and interacts with matter like a
particle
◼ Light traveling exhibits wavelength and
frequency
◼ Light interacting exhibits quanta