How to use the lensmaker's equation calculator
Find the focal length of a lens from its curvature and material. Enter refractive index, radius r1, radius r2 (negative if biconvex) in the fields above; the focal length is computed instantly and plotted on the chart so you can see how the answer changes.
Formula
Variables
- n — Refractive index
- R1 — Radius R1 (cm)
- R2 — Radius R2 (negative if biconvex) (cm)
Worked example
With refractive index = 1.5, radius r1 = 20 cm, radius r2 (negative if biconvex) = -20 cm, the calculator gives focal length ≈ 20 cm.
Frequently asked questions
What is the formula for the lensmaker's equation calculation?
Focal length (cm) = 1 / ((n-1) × (1 / R1-1 / R2)). Here n is refractive index, R1 is radius r1 (cm), R2 is radius r2 (negative if biconvex) (cm).
What inputs do I need for the lensmaker's equation calculator?
You need refractive index, radius r1 in cm, radius r2 (negative if biconvex) in cm. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use this calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.