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Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 xx 10^8 m/s., the focal length of the lens is ...... |
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Answer» 15 cm `=(3xx10^ 8)/(2xx10^8)=3/2` `THEREFORE n=1.5` (2) From right angle TRIANGLE, `R^2=(R-0.3)^2+(3)^2` `therefore R^2=R^2-0.6R+0.09+9` `therefore0.6R=9.09 therefore R=15.15" cm "therefore R=15` cm (3) From lens maker.s formula, `1/f=(n-1)((1)/(R_1)-(1)/(R_2))` `1/f=(1.5-1)((1)/(INFTY)-(1)/(-15))` `therefore 1/f=1/2xx(1)/(15)=(1)/(30)" "therefore` f=30 cm 
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