Index of Refraction Calculator
Table of Contents
1. Index of Refraction Calculator
Enter focal length and the two surface radii. The calculator rearranges the lens maker equation to solve for n, which helps identify or verify an optical material.
Index of Refraction Calculator
Enter focal length (f), R₁, and R₂ to calculate n
Formula
1/f = (n-1)(1/R₁ - 1/R₂)
Positive for convex, negative for concave
Positive for convex, negative for concave
Positive for converging, negative for diverging
2. Index of Refraction vs Refractive Index
These terms are interchangeable in optics. Both describe how much a material slows and bends light relative to vacuum.
Definition
n = c / v
c = speed of light in vacuum, v = speed of light in the material
3. Formula for n
Rearranging the lens maker equation for the index of refraction gives:
Index of Refraction Formula
n = 1 + 1 / [f × (1/R₁ - 1/R₂)]
This is the same relationship used by the refractive index calculator, but the page language matches users searching for the American phrasing index of refraction.
4. Typical Material Values
Common optical glasses
- BK7: 1.5168
- Crown glass: 1.52
- Flint glass: 1.62
- Dense flint: 1.70 to 1.80
Other materials
- Acrylic: 1.49
- Polycarbonate: 1.58
- Water: 1.33
- Diamond: 2.42
5. Worked Example
Given: f = 0.10 m, R₁ = 0.10 m, R₂ = -0.10 m
1/R₁ - 1/R₂ = 10 - (-10) = 20
n = 1 + 1 / (0.10 × 20)
n = 1 + 0.50
n = 1.50
That result is consistent with common crown glass. You can compare it against the materials guide or test the inverse workflow with the focal length calculator.
6. FAQ
Is index of refraction the same as refractive index?
Yes. They are two names for the same optical property, usually written as n.
What inputs do I need for an index of refraction calculator?
You need focal length plus the first and second surface radii, all expressed in the same unit system.
What is a realistic result for optical glass?
Most common optical glasses fall between about 1.48 and 1.80, while plastics are often around 1.49 to 1.59.
When should I use this page instead of the refractive index page?
Use this page if your target keyword or user wording is “index of refraction.” Functionally, both pages solve the same variable n.
Related Tools & Guides
Refractive Index Calculator
The same tool family with the standard optics wording.
Materials Guide
Compare your calculated value with common optical materials.
Focal Length Calculator
Run the inverse calculation when n is already known.
Radius of Curvature Calculator
Solve the missing lens surface after verifying n.
Focal Length & Refractive Index
See how these two variables move together in lens design.
Lens Maker Formula
Review the parent equation behind the calculator.