WAVE PARTICLE DUALITY Principles of Physics

Is light a wave or a particle??  Isaac Newton said light is a particle  Christian Huygens (explained diffraction) said light is a wave  Thomas Young (proved light diffracts and interferes) said light is a wave  James Clerk Maxwell (related electricity and magnetism through a set of complex equations) said light is electromagnetic radiation (wave)  Hertz (produced radio waves) supported Maxwell  Einstein said light is a particle

Einstein’s Particle theory  Light is made up of concentrated bundles of electromagnetic energy, which are also called quanta of electromagnetic energy

Quantum  An elemental unit  Smallest amount of something  Plural = quanta Example: Money – Each denomination is a whole number multiple of the smallest Smallest denomination: penny 1 dollar = 100 pennies 1 quarter = 25 pennies

Quantum Example  Mass of a pure gold ring = whole number multiple of the mass of a gold atom  Therefore, gold is quantized  In fact, all matter is quantized

Photons  A quantum of light is called a photon  Photons move at one speed: 300,000,000 m/s  Light is not emitted continuously  Instead it is a stream of photons each with its own energy

Photoelectric Effect  Proof of the existence of photons  Einstein won the Nobel Prize for this  Electrons can be ejected from certain materials when light shines on them  Energy is transferred from the photon to an electron

Photoelectric Effect Higher frequency allows the ejected electrons to have more energy when they leave Higher amplitude (brighter) light allows more electrons to be ejected, but has nothing to do with the energy of the individual electrons

Photoelectric Effect Photoelectric Effect - Light, Quantum Mechanics, Photons - PhET

Uses of Photoelectric Effect  Solar power  Photons eject electrons from photosensitive materials. The electrons collect and flow in wires creating a current)  Electric eyes (they can be used to open doors automatically)  A beam of light shines across an opening to a detector. The beam of light maintains a current until it is blocked by you coming to the door. When the detector senses the change in current it open the door.

Wave Model vs Particle Model WaveParticle 1. Reflection_______ _______ 2. Refraction _______ _______ 3. Diffraction _______ _______ 4. Interference _______ _______ 5. Photoelectric Effect _______ _______ XX X X X

Particles as Waves  All matter and energy have both wavelike and particle-like properties  Wavelike properties of matter are not normally seen unless you look at very fine details  Ordinary-size objects have wavelengths that are much too small to be measured. Therefore, a batter does not have to worry about the wavelength of a baseball.