Thin-Film Interference When any wave encounters an interface between mediums, there is always a reflection backward from the interfacewhether or not part of the wave continues into the new medium. Consider a thin film of oil on the surface of a puddle of water, as is often seen in parking lots after rain showers. Compared to the air above or the water below, the layer of oil is very thin. There are two interfaces: air/oil and oil/water. Light coming nearly straight down will partially reflect off both interfaces. (Large angles of incidence would refract too much, ruining the effect.) For certain wavelengths of light, the two reflected rays will add constructively, reinforcingeach other’s amplitudes. Since the reflected ray in the oil had to traverse its thickness ltwice, the condition for such constructive interferenceis 2l= Nλ, where Nis any integer greater than zero. This equation ensures that the phase shift between the two
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