Principal Axis - Line which passes through the center of a lens

Principle focus - Point at which rays of light travelling parallel to the principal axis intersect the axis and converge

Foal length - Distance bw center of lens and principle focus

Convex -

Parallel rays are brought to a focus

More curved the lens → Shorter focal length

Concave -

Parallel rays diverge

Principle focus - Point from which the ray appears to diverge from

Real And Virtual Image -

Real -

1. Image formed when light rays converge and can be projected onto a screen
2. Inverted [Always]