Inverse Square Law
The inverse square law states that the power density of radiation from a point source falls off in proportion to 1 / r², where r is distance from the source.
What is Inverse Square Law?
The inverse square law describes how the intensity of radiation from a point source decreases with distance. For a source radiating equally in all directions in free space, the power crossing a sphere of radius r is spread over a surface area of 4πr². Power density (W/m²) therefore falls as 1/r².
The practical consequence: doubling the distance from a source reduces power density to one quarter; tripling the distance reduces it to one ninth. Distance is the most effective single factor in reducing exposure to a radiating source.
The law applies cleanly only to true point sources radiating in free space, in the far field (typically more than a few wavelengths from the source). Several real-world conditions modify the relationship:
In the near field — within about one wavelength of the source — the field structure is dominated by reactive components and does not follow 1/r². For low-frequency sources, near-field falloff can be steeper (1/r³ for a magnetic dipole, for example).
Directional antennas concentrate energy into a beam, so on-axis power density falls more slowly than 1/r² for a stretch before reverting to inverse-square far from the antenna.
Reflections from walls, floors, and other surfaces add a constant or slowly-varying background that prevents indoor measurements from following the law cleanly.
Despite these caveats, the inverse square law is the right first-order intuition for everyday RF exposure: moving a router across the room reduces exposure dramatically; holding a phone to your ear instead of using a speakerphone increases it sharply.
Why does Inverse Square Law matter?
What does the inverse square law say?
The intensity of radiation from a point source falls in proportion to 1 over the distance squared. Twice the distance, one quarter the intensity.
Does the inverse square law apply to Wi-Fi and cellular signals?
Approximately, in the far field. Near a router or phone antenna the near-field structure dominates, and reflections in indoor environments add a background that does not fall off cleanly with distance.
Why does distance matter so much for EMF exposure?
Because the inverse square relationship makes distance more powerful than most other interventions. Doubling distance from a source achieves a 75% reduction in power density without any shielding at all.
How RADIHALT relates to Inverse Square Law
RADIHALT designs EMF protection blankets built around woven copper-nickel Faraday fabric. The terminology on this page — from Faraday-cage physics through attenuation figures and ICNIRP exposure limits — is what underpins the engineering and the claims we publish about our products.
We try to keep our marketing language tied to the same vocabulary regulators and physicists use. If a definition on this page conflicts with anything on a RADIHALT product page, the glossary entry is the source we hold ourselves to.
Related terms
Power Density (W/m²)
Power density is the amount of electromagnetic power passing through a unit area perpendicular to the direction of propagation, expressed in watts per square meter (W/m²).
Radio Frequency (RF) Radiation
Radio frequency radiation is the portion of the electromagnetic spectrum between about 3 kHz and 300 GHz, used by Wi-Fi, cellular, broadcast, and radar systems.
Specific Absorption Rate (SAR)
Specific Absorption Rate is the rate at which the human body absorbs radio frequency energy, expressed in watts per kilogram (W/kg) of tissue.
From definitions to a real shielding blanket.
RADIHALT applies the physics on this page in a portable, washable copper-nickel Faraday blanket. Starting at $22.