Obtain an expression for focal length of a combination of thin lenses in contact.

Obtain the equivalent focal length of combination of thin lenses placed in contact,

Derive the expression for the equivalent focal length of a combination of two thin convex lenses in contact.

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.

Derive the expression for effective focal length of two thin lenses kept in contact.