Lens Formula Calculator

Lens Type

Convex (+)

Concave (−)

Focal Length |f| (cm) 20 cm

Object Distance |u| (cm) 40 cm

Real | Inverted | Diminished

Object dist u

—

Image dist v

—

Focal length f

—

Magnification m

—

1/v − 1/u

—

1/f

—

Ray Legend

━━ Parallel ray → through F

━━ Ray through centre

━━ Ray through F → parallel

Key Formulae

1/v − 1/u = 1/f

m = −v/u = h_i/h_o

Convex: f > 0

Concave: f < 0

Real image: v > 0

Virtual image: v < 0

Sign convention (New Cartesian): distances measured from optical centre. Object always on left (u < 0). Real images form on opposite side (v > 0), virtual images on same side (v < 0). Convex lens has positive f, concave has negative f.