Parallel ray → through F′
Through centre
Through F → parallel
Object
Image
Key Concepts
Principal Axis
The horizontal line through the optical centre. All measurements are taken along or parallel to this axis.
Focus (F & F′)
Convex: rays converge at F′. Concave: rays diverge as if from F. Both foci are equidistant from the lens.
Real vs Virtual
A real image forms where rays actually meet and can be projected on a screen. Virtual images are seen through the lens.
Magnification (m)
m = v/u. Negative m → inverted image. |m|>1 → magnified, |m|<1 → diminished.
Power (P)
P = 1/f. Convex lenses have positive power, concave lenses have negative power.
Convention
New Cartesian: distances measured from optical centre. Incident light travels left→right. Distances right of lens are positive.