Snell's Law Calculator
Calculate the angle of refraction using Snell's law from the refractive indices and the angle of incidence, with total internal reflection detection.
Last reviewed by the Radiatus Cloud team
Calculate the refraction angle of light using Snell\u2019s law.
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Calculate refraction with Snell's law
Snell's law describes how light bends when it passes between two media of different optical density, such as from air into water. It states that the refractive index of the first medium times the sine of the angle of incidence equals the refractive index of the second medium times the sine of the angle of refraction. This calculator solves for the refraction angle, and detects total internal reflection, which occurs when light travelling into a less dense medium exceeds the critical angle and is reflected entirely instead of refracting.
The angles are measured from the normal, the line perpendicular to the surface.
Light and lenses
Refraction is why a straw looks bent in a glass of water, how lenses focus light, and the basis of optical instruments from spectacles to microscopes and fibre-optic cables. Light bends toward the normal when entering a denser medium and away from it when entering a less dense one. Total internal reflection is what keeps light trapped inside optical fibres, enabling high-speed communication.
Common refractive indices are about 1.00 for air, 1.33 for water and 1.5 for glass. The calculator flags when total internal reflection occurs and reports the critical angle. All calculation happens locally in your browser.
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Frequently Asked Questions
What is Snell\'s law?
It relates the angles and refractive indices at a boundary: the index times the sine of the angle is equal on both sides of the surface.
What is total internal reflection?
When light moves into a less dense medium beyond the critical angle, it cannot refract and is entirely reflected back, as in optical fibres.
Which way does light bend?
Light bends toward the normal when entering a denser medium with a higher refractive index, and away from the normal when entering a less dense one.
What are common refractive indices?
About 1.00 for air, 1.33 for water and roughly 1.5 for glass. Denser media have higher indices and bend light more.
How are the angles measured?
From the normal, the imaginary line perpendicular to the surface at the point where the light hits it, not from the surface itself.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter the two refractive indices and the angle of incidence.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.