A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive index `mu_(1)=4//3` and `mu_(2)=6//5` and R = 40 cm is the radius of curvature of the curved surface of the lenses, then the focal length of combination (in meters) is.

Answer

3

Explanation

The equivalent focal length (f) of the given lens as shown in figure is given by
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))`
`(1)/(f)=(mu_(1-1))((1)/(oo)-(1)/(-R))+(mu_(2)-1)((1)/(-R)-(1)/(oo))`
`=((mu_(1)-1))/(R)-((mu_(2)-1))/(R)=(mu_(1)-mu_(2))/(R)," "f=(R)/(mu_(1)-mu_(2))`