A plano convex lens fits exactly into a plano concave lens.
Their plane surfaces are parallel to each other. If lenses are
made of different materials of refractive indices μ₁ and μ₂
and R is the radius of curvature of the curved surface of the
lenses, then the focal length of the combination is [2013]
(a) R/2(μ₁ - μ₂)
(b) R/(μ₁ - μ₂)
(c) 2R/(μ₂ - μ₁)
(d) R/2(μ₁ + μ₂)
Answers
Explanation:
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Plano-Concave Spherical Lenses
Plano-concave lenses diverge a collimated beam from a virtual focus and are commonly used in Galilean-type beam expanders. Given their negative focal length and negative spherical aberration, plano-concave lenses can be used to balance out aberrations of other lenses in the system. Thorlabs provides plano-concave lenses fabricated from N-BK7, N-SF11, UV-fused silica, calcium fluoride, and zinc selenide. N-SF11 offers a usage range that is slightly shifted to the red compared to N-BK7. However, N-SF11 glass is associated with more dispersion because it has a lower Abbe Number than N-BK7 glass. For more information on these substrates, please visit our Optical Substrates