What is the power of a combination of lenses?

What is power?

What is power?

What is power factor?

Two lenses of powers, 15D and +5D are in contact with each other forming a combination lens. What is the focal length of the combination?

Two lenses 3D and -1.5 D were put in contact. What is the focal length of the combination?

Two identical equiconvex lenses of refractive index 3/2 and focal length 16cm are kept in contact. The space between the lenses is filled with water of refractive indext 4/3 . What is the focal length of the combination?