One surface of an equi convex lens, having spherical surfaces with radii 8cm, is silvered. The effective focal length of the lens will be
Answers
effective focal length of lens would be 2.67 cm.
radius of each spherical surface of equi - convex lens is , R = 8cm
a/c to question, one of the surface of equi convex lens is silvered. it means if an object is placed in front of lens, two times refractions and one times reflection will happen to make final image.
so, 1/F = 2/f_L + 1/f_M
where f_L is power of lens and f_M is power of mirror.
first we have to find power of equi convex lens.
1/f_L = (μ - 1)[1/R1 - 1/R2]
= (μ - 1)[1/8 + 1/8 ]
= (μ - 1)/4
now, 1/F = (μ - 1)/4 + 1/(R/2)
= (μ - 1)/4 + 1/4
= μ/4
F = 4/μ
if refractive index of glass is 1.5
then, F = 4/1.5 = 8/3 = 2.67 cm
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