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SC PHYS 202 - Light Waves

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Phys 202 1nd Edition Lecture 21Outline of Last Lecture I. Review of conceptsOutline of Current Lecture II. Speed of lightIII. Light Equations:a. Forceb. Intensityc. Radiation PressureIV. ReflectionV. Refraction Current LectureMaxwell’s equations consist of Gauss’s law of magnetostatics and electrostatics, Ampere’s law, coulombs law. These equations tell us that light is a wave. Speed of light:Essentially, the spacial variation of the electric field is consistent with the temporal variation of the magnetic field. The constant of the equation(c) is µ0E0 which is c=(1/ √µ0E0)=299,792,458m/s or 3x108m/s. This is the velocity or speed of light. The velocity of light will always be the same anywhere.The energy of the light is equal to the momentum of the light times the constant or speed of light. The equation for the energy of light then is:U=PcLight Equations:Remember Einstein’s theory of relativity is represented by E=mC2If C is constant, as it is the speed of light, then the mass is just energy, and vice versa. This means that light at rest has no mass, and therefore there is no such this as “stationary light”. Light must always be in motion.Force exerted by light:Light(c) will exert a force equal to;These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Δp/Δt = (Δpc)/(Δtc) = (Δuc/Δtc2) = (Δu/Δt)(1/c) = Power/cIntensity or irradiance:Intensity(i) is equal to power per unit area of light (i=F/A), similar to the way that pressure is equal to force per unit area. This, in turn, means that intensity is equal to the total energy divided by volume times the constant(c) (i=(U/v)c) which is equal to uc where u is energy density.Radiation Pressure:A planet close to the sun will experience a pressure due to the light of the sun. This is the radiation pressure(P). This pressure is equal to force over unit area (P=F/A). For something that absorbs light, the pressure is equal to the energy density over the constant (Δp=U/c). For something that reflects light, the pressure is equal to twice the energy density over the constant (Δp=2(U/c)).For something like the color black, which is totally absorbing, the force of the light is equal to the intensity times area over the constant (F=i A/c), and the pressure will be intensity over the constant (P=i/c). For something like white which totally reflects light, we just multiply each of these equations by 2to get the force and pressure; (F=2iA/c) and (P=2i/c).Reflection:If something is looking at itself in a mirror, the light traverses from the mirror in a triangle as in the figurebelow. The axis that defines the plane of light is the “normal”, denoted by the dashed line. The angle from the normal to the lower axis is the angle of incidence (φi) and the opposite angle is the angle of reflection (φr). Both angles will be equal to one another. All reflected images are considered virtual. Θr normalRefraction:If light moves from air into water, the path of the light will be refracted once it enters the water, as shown in the picture below. θi Airφiθ2 WaterSnell’s rule measures the refraction of the light in the water. The equation for Snell’s rule is nsin(θi)=n2sin(θ2) Where θi is the angle at which the light hits the medium and θ2 is the angle of the refracted light throughthe medium. n defines the medium that light travels through (like water) and is equal to the velocity of light in the vacuum over the velocity of light. n is always greater than or equal to one. n of water is equal to 1.33, n of glass is equal to 1.78, and n of diamond is about 4. n is the index of


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SC PHYS 202 - Light Waves

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