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When light passes from air into glass or water it bends towards the normal. Snell's law says the ratio sin i ÷ sin r is constant for a given pair of materials; for light entering from air (or a vacuum) this ratio is the refractive index n of the material.
Enter the two angles, both measured from the normal. The calculator gives n and the speed of light in the medium, v = c ÷ n.
| Angle of incidence (i) | 45 degrees |
| Angle of refraction (r) | 28 degrees |
| Refractive index n | 1.50618 |
| Speed of light in the medium (m/s) | 199,042,000 |
| sin i | 0.707107 |
| sin r | 0.469472 |
n = 1.50618, about the refractive index of glass; light travels at 199,042,000 m/s inside it.
Try this example →| Angle of incidence (i) | 40 degrees |
| Angle of refraction (r) | 28.9 degrees |
| Refractive index n | 1.33005 |
| Speed of light in the medium (m/s) | 225,400,000 |
| sin i | 0.642788 |
| sin r | 0.483282 |
n = 1.33005, the refractive index of water.
Try this example →n₁ sin θ₁ = n₂ sin θ₂. For light entering a medium from air, n = sin i / sin r.
How much a material slows and bends light: n = speed of light in vacuum ÷ speed in the material.
Not for ordinary materials and visible light; every medium slows light down.
No, it is a ratio of two speeds.