By: Andrew Disharoon, Nathan Robertson, Nigel Long, Brandon Huggins

• States:

• The idea that light is a wave motion

• Light will go in every direction from a light source

• Light spreads throughout the medium

Wave Theory of Light

http://physics.tamuk.edu/~suson/html/4323/intro.html

• Huygens’ Principle – “each point on the leading surface of a wave disturbance may be regarded as a secondary source of spherical waves, which themselves progress with the speed of light in the medium and whose envelope at later times constitutes the new wave front”

Wave Theory Allowed

http://physics.tamuk.edu/~suson/html/4323/geo

metry.html

• Newton came up with this theory

• Light is made of small particles

• Photons obey same laws of physics for masses

• Photons are so small that when two particle beams intersect they don’t rebound off of each other

Particle Theory of Light

http://galileo.phys.virginia.edu/classes/609

.ral5q.fall04/LecturePDF/L20-LIGHTII.pdf

• Later Max Planck said that particles have some wave like properties

• This explains the wave-particle duality

• Stating that light acts like waves in propagation, interference, and diffraction

• But like particles when reacting with matter

• Because of Planck quantum mechanics was created

Particle Theory Continued…

http://physics.tamuk.edu/~suson/html/4323/intro.html

• Planck's constant = 6.626068 10-34 m2 kg / s = h

• Planck’s constant was the result of experimentation

Planck’s Constant

• Diffraction- particles don’t change direction as they pass by a barrier.

• As the front of the wave passes over an object, the wavelets following curve around the object.

• This is why the edge of the shadow is not sharp and precise.

Diffraction

Light Diffraction experiment Fuzzy outline of shadow

Diffraction

• Diffraction is also used for finding the atomic structure of certain crystals.

Diffraction

• The amount diffracted light becomes less and less intense as the angle increases.

Diffraction

What is the Photoelectric Effect?

• The Photoelectric Effect is when electrons are emitted from matter (usually metal) when light is shined on them.

• This is because they absorb the energies in the light.

The Photo Effect and The Wave Theory

• This Effect was discovered by Heinrich Hertz in 1887. At that time, the accepted theory of light was Maxwell’s wave theory.

• His theory was that light was a wave.

The Photo Effect and The Wave Theory

• If light was a wave, then the intensity (amount) of light shone on metal would dictate the energy of the emitted electrons.

• To test this, Philip Lenard did an experiment with the Photoelectric Effect.

• But he found that the amount of light did not do anything to the energy of the emitted electrons.

The Experiment

• Philip Lenard put two pieces of metal in a vacuum. They were part of a circuit, as shown in the illustration.

The Experiment• The plate opposite the plate that the light would shine on was negatively

charged, so only very energetic electrons would reach the other side.

The Experiment

• When he shined high-frequency light on the metal, electrons were released, completing the circuit.

The Experiment

• But the low-frequency light did not have enough energy to get past the negatively-charged plate and contribute to the current

Conclusion

• Using this, Albert Einstein hypothesized that light was made of quantized photons, which explains why the frequency of the light affected the energy of the electrons.

Conclusion

• Einstein won the Nobel Prize by saying that each photon energized each electron. He won the Nobel for this hypothesis.

Photons

• Shot through one slit

• Create a single band on the detector plate

• Consistent with a particle pattern

• Shot through two slits

• Creates a pattern on plate

• consistent with a wave pattern

Double Slit Experiment

The photons could be interacting like particles in a wave.

Double Slit Experiment

Single

Photon

There can’t be any interference from the other photons so it should not make a wave pattern but

it does.

Double Slit Experiment