Snell's Law Calculator
Enter the two refractive indices and the angle of incidence to find the refracted angle using Snell's law, with the critical angle and total internal reflection.
How to use the Snell's law calculator
- Enter the refractive index of the first medium (n1).
- Enter the refractive index of the second medium (n2).
- Enter the angle of incidence and read the refracted angle.
Examples
Air to glass at 30 degrees
n1 = 1.0, n2 = 1.5, angle = 30 deg
refracted angle = 19.47 deg
Water to air past the critical angle
n1 = 1.33, n2 = 1.0, angle = 60 deg
total internal reflection (critical 48.75 deg)
Frequently asked questions
What is Snell's law?
Snell's law relates the angles either side of a boundary: n1 x sin(theta1) = n2 x sin(theta2), where n1 and n2 are the refractive indices of the two media.
What is the index of refraction?
It is how much a medium slows light compared with a vacuum. Air is about 1.00, water about 1.33 and common glass about 1.50.
What is the critical angle?
When light goes from a denser to a rarer medium (n1 greater than n2), the critical angle is asin(n2 / n1). Beyond it, light cannot pass through.
What is total internal reflection?
If the angle of incidence exceeds the critical angle, no light is refracted out and it all reflects back. This calculator flags that case for you.
Why is there no critical angle going air to glass?
A critical angle only exists when entering a less dense medium. Going from air into glass (n1 less than n2), light always refracts, so none is reported.
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