Apply the lensmaker's equation to derive focal length from radii and refractive index.
由折射率与两表面曲率半径,按 1/f = (n-1)(1/R₁ - 1/R₂) 计算透镜焦距。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Lensmaker's equation | 1/f = (n−1)[1/R1 − 1/R2 + (n−1)t ÷ (n·R1·R2)]; thin-lens drops the t term | Standard geometrical-optics formula |
| Sign convention | Front convex surface has R>0; signs set converging/diverging | Optical sign-convention practice |
| Biconvex example | n=1.5, R1=100 mm, R2=−100 mm gives f=100 mm (thin) | Worked substitution example |
| Thick-lens correction | When t exceeds ≈10% of f, use principal-plane (cardinal-point) analysis | Cardinal-point theory |