Snell's Law Calculator

Author: Henrick Yau
- 27 September 2026
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Calculators

Snell's Law describes how light refracts when passing between different media, such as air to water or glass to air. The law states that the ratio of the sines of the angles of incidence and refraction is equal to the ratio of phase velocities in the two media, or equivalently, to the inverse ratio of the indices of refraction.

This calculator allows you to determine the angle of refraction, critical angle, and other optical properties using Snell's Law.

What would you like to calculate?

? The refractive index of the first medium (where light comes from). For air at standard conditions, use approximately 1.000.
? The refractive index of the second medium (where light enters). For water, use approximately 1.333.
degrees
? The angle between the incident ray and the normal to the interface (0-90 degrees).

Display Options

? Shows how refractive indices and angles vary with wavelength (dispersion effect). Note that this is an approximate model.

About Snell's Law

Snell's Law (also known as the law of refraction) describes the relationship between the angles of incidence and refraction when light passes through the boundary between two different isotropic media, such as air, glass, or water.

The Mathematical Expression:

Snell's Law is mathematically expressed as:

n₁ sin(θ₁) = n₂ sin(θ₂)

Where:

  • n₁ is the refractive index of the first medium
  • n₂ is the refractive index of the second medium
  • θ₁ is the angle of incidence (measured from the normal)
  • θ₂ is the angle of refraction (measured from the normal)
Critical Angle and Total Internal Reflection:

When light passes from a medium with a higher refractive index to one with a lower index (n₁ > n₂), there exists a critical angle of incidence (θc) at which the refracted light travels along the boundary between the two media.

For angles of incidence greater than the critical angle, all light is reflected back into the first medium - a phenomenon called total internal reflection.

The critical angle is given by:

θc = arcsin(n₂/n₁)
Refractive Indices of Common Materials:
Material Refractive Index (approximate)
Vacuum 1.0000 (exact)
Air (at STP) 1.0003
Water 1.333
Ethanol 1.36
Crown Glass 1.52
Flint Glass 1.62
Diamond 2.42

Note: Refractive indices vary with the wavelength of light.

Key Applications:
  • Optical Systems: Lenses, prisms, fibre optics, and other optical components in cameras, microscopes, and telescopes.
  • Gemology: The brilliance of diamonds and other gemstones is due to their high refractive indices.
  • Fibre Optic Communications: Total internal reflection keeps light signals inside optical fibres.
  • Medical Imaging: Endoscopes and other medical optical instruments use principles of refraction.
  • Atmospheric Optics: Mirages, rainbows, and the twinkling of stars are all related to refraction.

Understanding Snell's Law

Snell's Law explains how light changes direction when it travels through different materials, like air and water. It tells us that the angle at which light enters a new medium depends on the properties of both materials. The law is often written as n₁ sin(θ₁) = n₂ sin(θ₂). Here, n represents the refractive index, and θ represents the angle relative to a line called the normal.

How the Snell's Law Calculator Helps You

This Snell's Law Calculator makes it easy to find key information about light refraction. You can calculate the angle of refraction, the angle of incidence, the critical angle, or even the refractive index itself. It’s a practical tool for anyone studying optics, whether you're a student or just curious about how light works.

Angles and refractive indices you can compute

  • Angle of Refraction: The angle at which light exits a new medium.
  • Angle of Incidence: The angle at which light enters a new medium.
  • Critical Angle: The angle at which total internal reflection occurs.
  • Refractive Index: A measure of how much the speed of light is reduced in a medium.

Using the Snell's Law calculator with materials

Using the Snell's Law Calculator is straightforward. You choose what you want to calculate, then input the refractive indices of both media along with the angle of incidence. If you’re unsure of the values, the calculator provides some common materials to select from, like air, water, and glass, making the process even simpler.

Precision, units and wavelength options

The calculator includes helpful options like decimal places for precision and the choice of degree or radian measurements. You can also see how wavelength affects the results, which is particularly useful for more advanced studies in optics. Each feature aims to give you the most accurate answers possible.

Visualising Light Refraction

As you use the calculator, it generates a diagram that illustrates how light refracts between the two media. This visualisation helps you better understand the concepts of incidence and refraction. Seeing how the angles change in a diagram can make the theory much clearer.

Real-World Applications of Snell's Law

  • Optical Devices: Lenses and prisms in cameras and glasses depend on Snell's Law.
  • Medical Equipment: Endoscopes use refraction to enhance imaging inside the body.
  • Gemology: The sparkle of diamonds relates to their refractive index.
  • Weather Phenomena: Rainbows and mirages are examples of light bending due to refraction.

Learn More About Snell's Law

Snell's Law is a fundamental principle in optics and plays a role in various scientific fields. Understanding how light behaves can lead to practical insights into technology, nature, and even everyday phenomena. This calculator is a great starting point for anyone wanting to explore the fascinating world of light and refraction.

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