Light – Wave or ParticleEvidence Supporting Wave Nature of Light• Reflection• Refraction• Diffraction• InterferenceWhere did we observe these phenomena?• PolarizationWhere did we observe this phenomena?
Light – Wave or ParticleEvidence Supporting Particle Nature of LightPhysics had yet to explain the following as of 1900:•Atomic Spectra Characteristic Color Pattern• Blackbody Radiation Absorption and Emission of Energy• Photoelectric Effect Photoemission of Electrons
Every element has its own signatureemission and absorption spectrum
Blackbody RadiationA blackbody is an object/system that absorbs all radiation in which it comes in contact. Blackbody Radiation is the radiation emitted from a blackbody based only on temperature.(usually associated with stars, we can measure temperature but not radiation levels)
Energy is QuantizedDefinitionEnergy comes in DISCRETE unitscalled quanta (photons).(Birth of Quantum Mechanics)
Photoelectric Effect Classical Physics vs. Quantum Mechanics Classical Quantum Mechanics Physics (light as a (light as a particle) wave)Whether electrons are The intensity of the The frequency of theejected depends on… light. light.The kinetic energy of The intensity of the The frequency of the ejected electrons light. light. depends on At low intensities, Occurs almost Takes time. instantaneously above electron ejection … a certain frequency. Proves duality of light.
Photoelectric EffectClassical vs. Quantum Mechanics• These observations support light being a particle rather than a wave.• It requires that electromagnetic energy be quantized as Max Planck proposed earlier in his explanation of blackbody radiation.• Albert Einstein extended Planck’s quantized energy to a stream of photons.
Work FunctionDefinitionThe minimum amount of energy required for anelectron to escape from a metal.The value of the work function depends ONLYon the metal.
Work FunctionThreshold FrequencyThe frequency of incident radiation (photons)below which there is no ejection of electronsfrom a metal.Symbol – ftUnits – Hz
Photoelectric Effect Explained: Light is going to hit the metal. If the frequency of the light is above the threshold frequency, then the loosely boundelectrons will be emitted (released from the metal). Metals with more loosely bound electrons will have a lower Threshold frequency, so it will require less energy to release the Electrons.