  # Interaction of Radiation with Matter: Reflection, Refraction, Polarization  Light dispersion through a prism. Light refraction through a slab with parallel surfaces. Example of polarizer and analizer effects on a non-polarized light beam.

 Q:    What are the basic principles which allow light to travel down a fiber optic wire even if the wire is curved? A:    Efficient transmission of light along optical fibers is based on the phenomenon of total reflection.   The condition for total reflection is that the incidence angle of the incoming light beam be larger than the characteristic critical angle of the fiber/coating system.   Remember that the phenomenon of total reflection is in fact a refraction phenomenon involving two media separated by a common interface.  The critical angle is defined as , where n2 is the index of refraction of the coating and n1 is the index of refraction of the fiber. Q:    There is a common experiment in which a pencil is placed in a glass of water and appears to be bent at the surface of the water.  Why does the pencil appear to bend? A:    Water and air have different indices of refraction.  Each medium slows down a beam of light differently.  As a consequence, light emitted by a single source travels through the two media with different speeds and in different directions.  The speed of light in each medium is determined by the index of refraction and the direction of travel in each medium is given by the Snell's Law (n1 sin 1 = n2 sin 2 ).  The pencil does not bend at the surface of separation between air and water; only the light emitted by each point of the pencil travels in different direction inside the two media. Q:     Since light travels so incredibly fast, how is it possible to accurately measure its speed? A:    Various ingenious experiments have been devised to measure the speed of light.  Some of them use astronomically large distances; others allow a light ray to travel back and forth a number of times between smaller distances on earth.  Both methods rely on a precise measurement of the time difference between emission and detection of a light signal.   Refinement of experimental techniques for measurement of the speed of light is still a challenge for physicists.

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### Equations

 Magnitudes of the electric field E and the magnetic field B in an  electromagnetic wave traveling along the x-axis E = Em sin (kx - t) B = Bm sin (kx - t) (where Em  and Bm are the amplitudes of E and B) Speed of any electromagnetic wave in vacuum Rate per unit area at which energy is transported by an electromagnetic wave (the Poynting vector S) Intensity of an electromagnetic wave Intensity of an electromagnetic wave at a distance r from a point source of power Ps Radiation force if the radiation is totally absorbed F = IA/c Radiation force if the radiation is totally reflected back along its original path F = IA/c Radiation pressure for total absorption pr = I / c Radiation pressure for total reflection back along path pr = 2I / c Intensity (I) of initially unpolarized light with intensity I0 after being passed through a polarizing sheet I = I0 Intensity (I) of initially polarized light with intensity I0 after being passed through a polarizing sheet at angle to the original direction of the light I = I0 cos2 Snell's Law of refraction. n1 sin 1 = n2 sin 2 Critical angle, if the angle of incidence exceeds the critical angle it will experience total internal reflection Brewster angle, in light strikes the boundary between two mediums at the Brewster angle it will be fully polarized by reflection # List of Topics

 Measurements Electric Potential Magnetism Electrical Circuits (AC) Optical Instruments: Mirrors and Lenses Electrostatics Capacitance Sources of Magnetic Fields Maxwell's Equations Interference Electric Fields Current and Resistance Magnetism in Matter Electromagnetic Waves Diffraction Electric Flux Electrical Circuits (DC) Electromagnetic Induction Interaction of Radiation with Matter: Reflection, Refraction, Polarization  ครั้งที่

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