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Asked in GSEB Board July 2016 · Lens maker's formula and power
Given: an equiconvex lens in air whose focal length equals the magnitude R of either radius.
Idea: equiconvex means the first face bulges towards the incoming light and the second away from it, so R₁ = +R and R₂ = -R.
1/(R₁)-1/(R₂) = 1/R+1/R = 2/R
Lens maker's formula: 1/f = (n-1)2/R, that is f = R/(2(n-1)).
Setting f = R gives 2(n-1) = 1, so n-1 = 0.5.
n = 1.5, the everyday value for crown glass.
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