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Unformatted text preview: 5.111 Lecture Summary #3 September 14, 2009 3.1 Reading for today: Sections 1.2 & 1.4 Reading for Lecture 4: Sections 1.5-1.6 Wave-Particle Duality of Radiation and Matter I. Electromagnetic (Light) Waves Let’s begin by describing the general properties of waves: (water waves, sound waves, EM or light waves) Waves have a periodic variation of some quantity Average level High density Low density SOUND WAVE WATER WAVE High level Low level Light waves come from the periodic variation of an electric field. The magnitude of the field oscillates along a direction in space. The electric field is the force field through which the Coulomb force operates – + ELECTRIC FIELD ELECTRIC FIELD + _ Wavelength λ Amplitude ELECTRIC FIELD + _ Wavelength λ Amplitude 5.111 Lecture Summary #3 September 14, 2009 3.2 The magnitude of the field also propagates in time. t=one period TIME FOR ONE CYCLE ≡ PERIOD x λ t=0 x ν ≡ FREQUENCY ≡ # cycles / sec PERIOD ≡ 1 ν sec λ x E(x) +- unit of frequency ≡ 1 Hz ≡ 1 / sec Let’s take ν = 1 Hz The oscillating electric field has an oscillating magnetic field perpendicular to it. This is what makes ELECTROMAGNETIC (EM) RADIATION Described mathematically as: E x, t ( ) = a cos 2 π x λ − 2 πν t ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ electric field position time The EM wave is a function of 2 variables, x and t. To visualize it, we hold 1 variable constant and plot it as a function of the other (at a fixed position as a function of time, or at a fixed time as a function of position) Also Polarization ( ˆ ε ): the direction of the electric field We’ll suppress the vector notation, but the field is a vector with a well defined direction....
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