If a convex lens of focal length 10 cm and a concave lens of focal length 50 cmare keptin contact with each other , (i) what will be the focal length of the combination ? (ii) What will be the power of the combination? (iii)What will be the behaviour ofthe combination ( behaviour as a convex lens / concavelens ) ?

If a convex lens of focal length 10 cm and a concave lens of focal len

1.	If a convex lens of focal length 10 cm and a concave lens of focal length 50 cmare keptin contact with each other , (i) what will be the focal length of the combination ? (ii) What will be the power of the combination? (iii)What will be the behaviour ofthe combination ( behaviour as a convex lens / concavelens ) ?
Answer» Solution :` f_(2) = - 50 cm` = - 0.5 m ( concavelens ) , f ( combination ) = ? P ( combination ) = ? (i) ` 1/f = 1/f_(1) + 1/f_(2) = 1/( 0.1 m) + 1/( - 0.5 m) = 10/m - 2/m = 8/m` ` f = 1/8 m = `0.125 m = 12.5 cm The focal LENGTH of the combination of the LENSES= 0.125 m= 12.5 cm. (ii) ` P _(1) = 1/f_(1) = 1/(0.1m) = 10 D, P_(2) = 1/f_(2) = 1/(-0.5 m) = -2 D ` ` therefore P = P_(1) + P_(2) = 10 D - 2D = 8 D ` ` P= 1/f = 1/( 0.125 m) = 8 D ` The power of the combination of the lenses = 8 D (iii) The focal length of the combination of the lenses is positive. This shows that the combination will behave as a convexlens.