Browse Definitions:
Definition

inverse-square law

The inverse-square law is a principle that expresses the way radiant energy propagates through space. The rule states that the power intensity per unit area from a point source, if the rays strike the surface at a right angle, varies inversely according to the square of the distance from the source.

Imagine a 40- watt lamp at the center of a spherical enclosure. The total power striking the surface of the sphere is 40 watts, no matter what the size of the sphere. Some people find this counter-intuitive, but it becomes obvious when we consider that all the radiated power (no more and no less) from the source must strike an enclosure of any size that completely surrounds the source. Thus, the power striking the interior of a sphere 10 meters across is the same (40 watts in this hypothetical example) as the power striking the interior of a sphere 100 meters across, 100 kilometers across, or 100,000 kilometers across. The power per unit area, however, does depend on the size of the sphere.

The surface area A (in meters squared) of a sphere having radius r (in meters) is given by:

A = (4 pi) r 2

where pi is the ratio of a sphere's circumference to its diameter, and is approximately equal to 3.14159. If the radius of a sphere is multiplied by some factor n , then the surface area increases by a factor of n 2 . This decreases the power per unit area by a factor of n 2 . Another way of saying this is that the power per unit area becomes n -2 times as great.

Increasing the diameter of a sphere from 10 to 100 meters makes it 10 times as large in diameter, and gives it 100 times the surface area. This cuts the light power per unit area from a lamp at the sphere's center by a factor of 100. The power landing on, say, one square centimeter of the sphere's interior becomes 1/100, or 0.01 times, as great, if the sphere's diameter grows from 10 to 100 meters. The law applies only as long as the point source is at the center of the sphere, so the rays from the source strike the sphere's surface at right angles.

Also see meter , square meter , and International System of Units ( SI ).

This was last updated in September 2005

Start the conversation

Send me notifications when other members comment.

SearchCompliance

• risk map (risk heat map)

A risk map, also known as a risk heat map, is a data visualization tool for communicating specific risks an organization faces.

• internal audit (IA)

An internal audit (IA) is an organizational initiative to monitor and analyze its own business operations in order to determine ...

• pure risk (absolute risk)

Pure risk, also called absolute risk, is a category of threat that is beyond human control and has only one possible outcome if ...

SearchSecurity

• FIDO (Fast Identity Online)

FIDO (Fast ID Online) is a set of technology-agnostic security specifications for strong authentication. FIDO is developed by the...

• cryptanalysis

Cryptanalysis is the study of ciphertext, ciphers and cryptosystems with the aim of understanding how they work and finding and ...

• Trojan horse (computing)

In computing, a Trojan horse is a program that appears harmless, but is, in fact, malicious.

SearchHealthIT

• vendor neutral archive (VNA)

A vendor neutral archive (VNA) is a technology that stores medical images in a standard format and interface, making them ...

• HITECH (Health Information Technology for Economic and Clinical Health) Act of 2009

The HITECH (Health Information Technology for Economic and Clinical Health) Act of 2009 is legislation that was created to ...

• Epic Systems

Epic Systems is one of the largest providers of health information technology, used primarily by large U.S. hospitals and health ...

SearchDisasterRecovery

• business continuity and disaster recovery (BCDR)

Business continuity and disaster recovery (BCDR) are closely related practices that describe an organization's preparation for ...

• business continuity plan (BCP)

A business continuity plan (BCP) is a document that consists of the critical information an organization needs to continue ...

• call tree

A call tree -- sometimes referred to as a phone tree -- is a telecommunications chain for notifying specific individuals of an ...

SearchStorage

• cloud storage provider

A cloud storage provider, also known as a managed service provider (MSP), is a company that offers organizations and individuals ...

• personal cloud storage (PCS)

Personal cloud storage (PCS) enables users to store data, photos, music, videos and other files on a local network-attached ...

• cloud SLA (cloud service-level agreement)

A cloud SLA (cloud service-level agreement) is an agreement between a cloud service provider and a customer that ensures a ...

SearchSolidStateStorage

• hybrid hard disk drive (HDD)

A hybrid hard disk drive is an electromechanical spinning hard disk that contains some amount of NAND Flash memory.

Close