The focal length of a plane glass is the distance between the surface of the glass and the point where the light rays passing through the glass converge to a single point.
The focal length of a plane glass can be measured by using a lensometer or by using the lens formula, which takes into account the curvature and refractive index of the glass.
The focal length of a plane glass is affected by the curvature of the glass, the refractive index of the glass, and the surrounding medium. It also depends on the angle at which the light rays enter the glass.
Focal length and thickness of a plane glass are two different properties. Focal length refers to the distance at which light rays converge after passing through the glass, while thickness refers to the physical distance between the two surfaces of the glass.
The focal length of a plane glass determines the size and position of the image formed by the glass. A shorter focal length will result in a larger and closer image, while a longer focal length will result in a smaller and more distant image.