The front Focal point of an optical system, has the property that any ray that passes through it will emerge from the system parallel to the optical axis. The rear (or back) focal point of the system has the reverse property: rays that enter the system parallel to the optical axis are focused such that they pass through the rear focal point.

The front and rear (or back) focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. An object an infinite distance away from the optical system forms an image at the rear focal plane. For objects a finite distance away, the image is formed at a different location, but rays that leave the object parallel to one another cross at the rear focal plane.

see wat tweety said is rit... definitely....

but understanding is .....

focal plane is a plane at which any sytem of parallel beam meet.. i mean its locus...

if the rays are parallel to principal axis thn they all r going to be definitely meeting at principal focus... but if it is not parallel to pric. axis but they all are parallel to each other they ud meet somewhre on focal plane....

as said focal plane passes thru principal focus... must.......

Cheers!!!!

It is simply a plane passing thru the focus and perpendicular to the principle axis.

Any parallel beam of light(bundle of light rays) passing thru a lens meets on the focal plane.

In the special case that the beam is parallel to the principle axis, then the rays meet on the focus itself.