d \h^ principal focus, andits distance from the lens is called \\\^ focal length of the lens. Obtainanother lens and find its focal length, then place the two together, andyou will find that the focal length of the combination is less than thatof either one alone. Look at the letters of your book… … Fig. 76. Foci. — The focal length of a lens depends upon itscurvature and the index of refraction of the material ofwhich it is composed. If both surfaces are equally curved,and if the lens is made of glass whose index is 1.5, theprincipal focus will be at the center of curvature. If raysof light…
Physics
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