1only first cassse was asked to bataya kyun nahin
Bohat type karna pada bekar main :P
but nevertheless if it helped u then its fine :)
If one face of a bi-convex lense with focal length f is silvered , what will be the new focal length ?
Also what if the lense was a plano convex lense??
the second part of the qsn was asked in wbjee..which i could not answer...[2]
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5 Answers
1When one surface of a biconvex lens is silvered then in this situation , Two times refraction through lens and one time reflection occurs
So power of combination
P = PL+Pm+PL = 2PL+Pm
Pm=-1fm , fm=R2 , PL = 1fL (fL is in metre )
for plano convex lens
two cases may arise
CASE-I
If plane surface is silvered
let radius of curvature is R and μ be its refractive index
P = 2PL + Pm
PL = 1fL = (μ-1)\left[\frac{1}{R}-\frac{1}{-\infty } \right]
\Rightarrow P_{L} = \frac{(\mu -1)}{R}
PM = -1fm = -1∞ = 0
hence P = 2(μ-1)R , f = R2(μ-1)
CASE - II
if curved surface is silvered
P = 2PL+Pm
P_{L} = (\mu -1)\left[\frac{1}{\infty } -\frac{1}{-R}\right] = \frac{(\mu -1)}{R}
Pm = -1fm = -1-R/2 = 2R
hence P = 2μR and f = R2μ
11thanks Manmay....
thanks for giving both the cases..though only the first case was asked in Wbjee....
kudos....
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111sry...but ya both of them helped me....thanks once again...