Two lenses of focal lengths `f_1` and `f_2`, are placed in contact. Obtain a general expression for effective focal length in terms of `f_1` and `f_2`

Combination of lenses are used in optical instruments to obtain the required power. Obtain an expression for the effective focal length of the combination of two thin convex lenses in contact.

Two lenses L_1 and L_2 are placed in contact as shown in figures. The focal length of each lens is 10cm.What is power of L_1

Two lenses L_1 and L_2 are placed in contact as shown in figures. The focal length of each lens is 10cm.What is effective focal length of combination

A convex lens of focal length .f is placed in a medium of refractive index equal to that of lens What happens to its focal length?