1Class 11Light / Electromagnetic radiation Speed of light Basics of electromagnetic radiation Particle/wave dualityI : The speed of light Light travels through a vacuum at a finite (butvery fast) speed First evidence obtained by Romer who was carefullytiming when the moons of Jupiter entered theshadow of Jupiter Found that these eclipses were 16.6mins “late” whenJupiter was on the far side of the Sun Interpreted as the extra time it takes for light tocross the diameter of the Earth’s orbit One way to obtain accurate measurement inthe laboratory is to use rapidly rotatingmirrors2Modern viewpoint… speed of light defined to bec=299 792 458 m/s3II : Basic nature of light Maxwell (1870) Starting from two important experimental results(Ampere’s Law and Faraday’s law of induction),Maxwell developed a unified theory of electricity andmagnetism (Maxwell’s equations). Electric and magnetic fields are just facets of aunified electromagnetic field Immediate prediction of Maxwell’s equation… wavesof electromagnetic energy can travel throughvacuum with a speed of 3.0x108m/s. He realized that he had just “discovered” light These waves were characterized by their wavelengthor frequency.Wavelength λ is the distance between two peaksSpeed c is the speed at which a peak moves through spaceFrequency ν (greek “nu”) is the number of peaks passing aparticular location per unit time4 We already knew that light was a wave from experimentsof Thomas Young (1803; the “Young’s slit” experiment) Wavelength determines color5 Young’s slit formula… bright peaks areat angles given byMaxwell’s equations putno restriction onwavelength of radiation…so e/m waves existoutside of the visibleband!6The Andromeda galaxy (M31)7The Andromeda galaxy (M31)8IR9III : Particle-wave duality We have seen that light can be understood aselectromagnetic waves… they undergo diffraction andproduce interference patterns BUT, light/electromagnetic-radiation can also behave asa stream of particles… Carries energy and momentum in discrete packets Can create a blip on a CCD chip This strange double-behavior is referred to as particle-wave duality The “light particles” are called photons - they arequanta of the electromagnetic field Properties of photons : Suppose that thecorresponding electromagnetic wave hasfrequency ν. Then… Photons has energy Photon has momentum Photon has angular momentumh is called