Question

Focal plane of a lens is a plane​

Answer 1

The distance from the centre of the lens to the focal point is the lenses focal length. A plane drawn perpendicular to the lens Axis at the focal point is the focal plane. Front object side the side of a lens where an object to be imaged is called the object side or front side of the lens.

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Answer 2

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For a defocusing system, the back focal point can be a virtual point located on the front side. The focal planes are defined as the planes perpendicular to the optical axis which contain the focal points. The focal points and planes can be found for any optical system, except if it is an afocal optical system.