Equations · equation
Inverse-Square Law of Intensity
Waves & Optics · 1604
- Equation
- I = P / (4π r²)
- Field
- Waves & Optics
- Symbols
- I intensity: power per unit area at distance r · P the source's total power · 4πr² the area of the sphere the power has spread over
- Year
- 1604
Significance
Light, sound and gravity spread out over a sphere whose surface grows as the square of the distance, so what reaches you falls as one over that square. Kepler stated it for light in 1604. It is why a star's apparent brightness gives its distance once its luminosity is known — the whole cosmic distance ladder — and why doubling the distance from a lamp quarters the light.
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