Chapter 16: Light
Particle (Corpuscular) Theory
Isaac Newton: 1643-1727
1. Rectilinear Propagation
2. Reflection
3. Refraction
Light consisted of very small mass
particles that travel at very high speed
Wave Theory
Christian Huygens: 1629 -1695
1. Rectilinear Propagation
2. Reflection
3. Refraction
Implies diffraction and interference,
which had yet to be discovered.
Young’s Double Slit Experiment
Leads to questions – What kind of
wave? How does it travel through
space? Is space empty?
James Maxwell: 1831-1879
Noted that electricity and light
travel at the same speed
Proposed a theory that light
was a type of electromagnetic
wave
Electric, Radio, Infrared, Visible, Ultra-Violet, X-Ray, Gamma
Maxwell concluded that all E-M waves travel at speed of light
How E-M waves are propagated remained a mystery! i.e. Do
they need a medium? If so, how does light travel from the
Sun to the Earth. Perhaps there is some unknown medium.
Scientists assumed that, like Sound, a medium was necessary.
They invented a, yet undiscovered, Aether, and tried to prove
or disprove its existence.
Michelson-Morley Experiment - 1887
Albert Michelson (1852-1931 )
Edward Morley (1838-1923)
Michelson and Morley expected to see a difference in the
speed of light depending on the relative motion of the
Earth as it passes through the Aether.
To their surprise the speed of light is independent of the
Earth’s motion i.e. no Aether! The implications are that light
travels through empty space!
The Photoelectric Effect
the emission of electrons from matter upon
the absorption of E-M such as U-V or x-rays.
Max Planck (1858-1947)
When E-M waves are incident on the “cathode”, electrons
are emitted and are forced to move to the “anode”
If the frequency of the E-M waves was decreased below a
certain value, the electrons stopped being emitted! Why?
Explanation of Photoelectric Effect
- Energy is not continuous
- Light comes in packages or
“quanta” of Energy called Photons
- The magnitude or size of the photon
is determined by its frequency.
- If the magnitude of the photon does
not meet the emission energy
requirements of the electron it
will not be emitted.
Albert Einstein (1879-1955)
It seems that Newton was right – light is a particle --- sort of!
Present Day Theory of Light
Dual Nature of Light:
light exhibits characteristics of waves and particles
at different times.
At the macroscopic level, light behaves as a wave.
Light (or any E-M photon) are propagated through
space by an alternating Electromagnetic Field. i.e.
they do not need a medium (actually, matter
hinders the transmission of light)
When light interacts with matter at the atomic
level, it acts like a particle, where the size of the
particle is determined by the frequency.
Quantum Theory
Max Planck, Albert Einstein, Niels Bohr
Niels Bohr (1985-1962)
The work of Planck and Einstein led Bohr to revise our ideas
about the atom i.e. that electrons exist in definite orbits and
need a definite amount of energy to change orbits