Measurement of focal length of lenses in contact and correction for their thickness and study of combination of lenses to reduce chromatic and spherical aberration .

Derive the equation for effective focal length for lenses in out of contact.

Two lenses of focal legths f_(1) and f_(2) are kept in contact coaxially. The resultant power of combination willl be

Two lenses of power +15 and - 5D are in contact with each other forming a combination lens.(a) What is the focal length of this combination? (b) An object of size 3 cm is placed at 30 cm from this combination of lenses. Calculate the position and size of the image formed.

Two lenses of power - 15 D and + 5 D are in contact with each other. The focal length of the combination is

Two lenses of power 3D and -1D are kept in contact. What is focal length and nature of combined lens?