By Zencalcy Editorial TeamLast reviewed About us
A smooth surface reflects light so that the outgoing ray leaves at the same angle at which it arrived, measured from the surface normal. That single rule explains mirror images, periscopes and the glare on still water.
Reflected angle from the angle of incidence.
θ_r = θ_i
θ_i is the angle of incidence and θ_r the angle of reflection, both measured from the normal. The incident ray, reflected ray and normal all lie in one plane.
| Symbol | Meaning |
|---|---|
| θ_i | Angle between the incident ray and the normal. |
| θ_r | Angle between the reflected ray and the normal. |
The reflected ray also leaves at 30° from the normal, on the other side. A ray that strikes at 0° — straight along the normal — returns along the same line.
Continue with the Combined Lens Power Calculator, Lens Maker's Equation Calculator, or the Lens Power Calculator — or browse more optics and light tools and all physics calculators.
From the normal. Measuring from the surface gives the complementary angle and is the most common mistake in mirror problems.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026