What is the condition for achromatism of two thin convex lenses put in contact with each other?

What are the uses of putting two lenses in contact with each other ?

Derive the expression for the power of two thin lenses placed in contact with each other.

The objective and eye pieces used in a compound microsciope are not single lenses but are the combinations of more than one lenses placed in contact with each other. Explain why?

What is the essential condition for addition of two vectors?

Fill in the blank Friction opposes the ……….between the surfaces in contact with each other.

What is the focal length of a combination of two concave lenses both of focal length 20 cm ?Is the system,a converging or a diverging lens?Ignore thickness of the lenses.