2054Lecture13

# 2054Lecture13 - Light Light Topics of Discussion Topics...

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Unformatted text preview: Light Light Topics of Discussion Topics Wave–Particle Duality Speed of Light Light as Waves Intensity of Light Polarization & Malus’ Law The Doppler Effect Wave–Particle Duality Wave–Particle Wave-particle duality says that light can be viewed as a particle or as a wave. viewed Wave Phenomenon: Interference and Diffraction. Diffraction. Particle Phenomenon: Photoelectric Effect and Compton Effect Effect The Speed of Light The Light travels in a vacuum at a speed of Light 2.997 × 108 m/s, and is given by 10 m/s c= 1 µ 0ε 0 Light as Waves Light Light is composed of oscillating electric and magnetic Light field that travel perpendicular to one another at a speed of light in a vacuum. The frequency f and wavelength λ and light in of light are related by λ = c/f. of Electromagnetic waves are transverse waves since the Electromagnetic electric and magnetic fields are perpendicular to the direction of the wave. direction Wave fronts are surfaces of constant phase and are E(x, t) = E0 sin (kx – ωt) kx B(x, t) = B0 sin (kx – ωt) kx separated by one wavelength. The wave front may be plane or a curved surface. The plane and spherical plane wave fronts are two important types of wave fronts. wave Intensity of Light Intensity The electric and magnetic energy density is the The electric magnetic energy contained in a region of space per unit volume, and is given by uE = ε0 E2/2 and uB = B2/2μ0. The total energy density (J/m3) iis given by s The total u = ε0 E2/2 + B2/2μ0. The total intensity of the light is given by The total S = cu = ε0 cE2/2 + cB2/2μ0 = P/A, where P is where cu cE the power that is delivered to a surface of area A. Polarization Polarization We call light linearly polarized if the We linearly vibrations of a transverse electromagnetic wave always occur along one direction. wave If the transverse electromagnetic wave If has no preferred (random) direction then the light is said to be unpolarized. unpolarized The principal axis of a polarizer is called The the transmission axis. transmission Three Types of Polarization Three Selective absorption occurs when only certain wavelengths of polychromatic light is absorbed by a polarizing material. by Reflection occurs when only certain wavelengths of polychromatic light is reflected from a polarizing material. polarizing Scattering occurs when light is scattered by the atoms or molecules of a polarizing material. atoms Malus’ Law Malus’ A polarizer-analyzer consists of a polarizing polarizer-analyzer material, called a polarizer, and a second polarizing material, called an analyzer, that is placed at an angle θ relative to the transmission axis of the polarizer. If the original average intensity enters the polarizer-analyzer is I0 then the final average intensity I is given by Malus’ Law. Law. I = I0 cos2θ The Doppler Effect The The frequency f0 of light measured by an The observer moving at a velocity v << c is << i s v f0 = f s 1 ± c where fs is the frequency of the light measured in where the source frame of reference. The positive sign applies if the source and the observer are moving towards one another. The negative sign applies if the source and the observer are moving away from one another. moving ...
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