Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi

Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi present the content wave characteristics, electrically charged particles and electromagnetic waves, effect on electric charges, accelerated charges (electrons, protons) produce,...
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Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan ThiQuantum Nature of Light (Quantum Optics) Pham Tan Thi, Ph.D.Department of Biomedical Engineering Faculty of Applied SciencesHo Chi Minh University of Technology Courtesy of N. Brunner and J. SimmondsLight behave like a wave or a particle (This is called the duality of the behavior of light)Wave CharacteristicsElectrically Charged Particles and Electromagnetic Waves Electrons have (-) charge Protons have (+) charge Both have electric fields +- attract ++ and -- repel • The changing position of a charged particle creates “waves” called electromagnetic waves • The electromagnetic waves travel through empty space eventually interacting with a distant charged particle. • Visible light is an electromagnetic wave. Magnetism(Effect on electric charges) Moving electric charges also produce magnetic fields. Example: electric current passing through a coil. Another interesting example: the Earth’ magnetic field is produced by the spinning of charges in the liquid metal core of the Earth. Conversely, magnetic field force charged particles to move…Accelerated Charges (electrons, protons) Produce (Ripples in the Electromagnetic Field)An electromagnetic wave iscomposed of two oscillating fields,and electric field and a magneticfield perpendicular to each other Antenna receives electromagnetic signal from TV stationWavelength of Electromagnetism means COLOR The Temperature Scale (Conversion from wavelength/energy to temperature)The scale mostly used in sciences, physics and astronomy is Kelvin. The unit is Kelvin (K). Thermal Radiation• The fundamental sources of all electromagnetic radiation (EMR) are electric charges in accelerated motion.• All bodies emit electromagnetic radiation as a result of thermal motion of their molecules. This radiation, called thermal Reference radiation, #1 of different is a mixture wavelengths. Reference #2• Thermal radiation is emitted by all objects An electric heating element emits primarily infrared radiation. But if its above absolute zero (-273.15°); but some temperature is high enough, it also of objects is in visible. emits a discernible amount of visible light. What is a BlackbodyA blackbody is an idealized object thatabsorbs all EMR that falls on it - noEMR passes through it and none isreflected (i.e. perfect emitter andperfect absorber).Because no light (visible EMR) isreflected or transmitted, the objectappears blackLeave appears green because greencolor is reflected to human eye Blackbody in Lab Experiment• An object of controlled temperature T contains a cavity, joined to the outside by a small hole. • If the hole is small enough, the radiation in the cavity comes to equilibrium in the walls.• The hole allows a small fraction of the radiation to pass to a spectrometer — the radiation coming out has the same spectrum as what is inside.• The radiancy is the power emitted per unit area per increment of wavelength and so has unit of Wcm-3 Blackbody in Lab Experiment• The spectral radiance from the hole is independent of the material used and only depends on the temperature.Rayleigh - Jeans Approximation for BlackbodyKirchhoff’s Law of Thermal Radiation in Thermal EquilibriumUltraviolet Catastrophe Quantum Theory1. Stefan-Boltzmann’s Law2. Wien’s Law3. Planck’s Theory4. Planck’s Formula Stefan - Boltzmann’s LawThe total emitted radiation (Mλ) from a blackbody is proportional to thefourth power of its absolute temperature. M = T4where σ is the Stefan-Boltzmannconstant, 5.6697 x 10-8 Wm-2K-4—> the amount of energyemitted by an object such as theSun or the Earth is a function ofits temperature—> This can be derived byintegrating the spectral radianceover entire spectrum Z 1 2⇡ 2 k 4 4 L= L d = 2 3T OR M = ⇡L = T 4 0 5c h Wien’s Displacement LawIn addition to computing the total amount of energy existing atheoretical blackbody such as the Sun, we can determine its dominantwavelength (λmax) based on Wien’s displacement law: k max = Twhere k is the is the Wien’sdisplacement constant = 2.898x 10-3 Km, and T is the absolutetemperature in Ki.e. there is an inverserelationship between thewavelength of the peak of theemission of a blackbody and itstemperature. Therefore, as the Sun approximates a 6000 K blackbody, its dominant wavelength (λmax) is 0.48 µm Planck’s Quantum TheoryMax Planck found a correct law for the black body radiation byassuming that each oscillator can only exchange energy in discreteport ...