Light
We have observed light
1. travels in straight lines
2. reflects
3. refracts
It also
4. transmits energy from
one place to another
Two theories could explain
these phenomena.
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Thanks to friends at Texas A&M U for help with the slides
What were the two theories of light in the 1600s
and who were their main advocates?
•
•
•
•
•
A.
B.
C.
D.
E.
Corpuscular-Newton and Wave-Huygens
Corpuscular-Huygens and Wave-Newton
Gravity-Newton and String-Weinberg
Force-Newton and Wave-Huygens
Wave-Aristotle and Gravity-Newton
The WAVE THEORY,
THEORY,
advocated by
Christian Huygens
and Robert Hooke,
Hooke,
said that light was a wave.
The PARTICLE (corpuscular) THEORY,
THEORY, advocated
by Isaac Newton and later by Pierre Laplace,
Laplace, said
that light was
made up of a
stream of tiny
particles called
corpuscles.
The more popular theory was the particle
 theory
of the reputation
Isaac
Newton.
because 
  of


.
.
The debate among the two sides continued
through the mid 1800’
1800’s.
Newton’
Newton’s particle theory could easily explain the
straight line travel of light, reflection, and energy
transmission, but had trouble explaining refraction.
1801  interference of light was discovered
1816 - diffraction of light (actually observed
in the 1600’
1600’s but not given much significance)
was explained using interference principles
Newton’
Newton’s explanation of refraction required that
light must travel faster in water than in air.
air.
Huygen’
Huygen’s wave theory could easily explain reflection,
energy transmission, and refraction, but had difficulty
explaining the straight line travel of light.
The wave theory’
theory’s explanation of refraction
required that light must travel
slower in water than in air.
air.
Neither phenomena could be explained
satisfactorily by the particle theory.
The final blow to the particle
theory came in 1850 when
Jean Foucault discovered that
light traveled faster in air
than in water.
water.
It was then widely accepted that light was a wave,
but what kind of wave?
In 1865, James Maxwell
developed ideas began by
Michael Faraday into a
series of equations that
proposed the electromagnetic wave theory.
theory. It
said that light was a type of e/m wave: a periodic
disturbance involving electric and magnetic forces.
forces.
foucault light demo
Foucault put a tube of water in the light beam and
showed that light slowed down in the water!
Newton was wrong!
The whole universe is composed of SHOs
SHO is system with linear restoring force to original
shape/position
Mass/spring
Frequency f =
1 

T 2
ms 
k
m
g
L
c
c = λƒ
Always true: Light-transverse
Sound-longitudinal
Any wave
Most waves need a medium-Except light

Types of Waves – Transverse

X direction (space/not time)
Longitudinal or compression
wave

Wave motion-Wave velocity c
c
t
Oscillators make Waves
All waves carry energy and momentum
Each part of spring moves
To wave motion
Like AM radio
Pendulum
p 
In 1885,
1885, Heinrich Hertz
experimentally confirmed
the e/m theory of light.