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As the figure shows, if the angle of incidence I (the arIgle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. Ifthe angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. Note that the critical angle is a property of the substance, and its value differs from one substance to another. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding ofless dense glass or plastic. The difference in density ofthe two materials must be such that a beam of light moving through the core is reflected off the cladding instead ofbeing refracted into it. See Figure
