Inverse Square Law Simulation: Light, Fields & Flux
Use this inverse square law simulation to explore light intensity, radial fields and conserved flux.
How to use this inverse square law simulation
This free inverse square law simulation helps GCSE and A-Level Physics students explore light intensity, electric fields and gravitational fields. Move the two comparison surfaces and watch how the same beam spreads over a larger area. Change the radiant power or light spread, then reveal the area and intensity calculations. Drag to rotate the view, or use the view and zoom buttons.
Why does doubling the distance quarter the intensity?
In the square beam, both side lengths are proportional to the distance from the source. Doubling the distance therefore makes each side twice as long and the area four times larger. The same radiant power passes through both surfaces, so the average intensity falls to one quarter. For an isotropic point source, the power spreads across a sphere of area 4πr², giving I = P/(4πr²).
Inverse square law simulation investigations
Use the inverse square law simulation to record several readings. Select Investigate & graph and move area 1 between measurements. Compare intensity against distance with intensity against 1/r². The second graph is a straight line through the origin for this ideal model. You can reveal the theoretical curve or export your measurements as a CSV file. Keep power and spread constant during each investigation.
Connect radial fields, force and flux
The Radial fields tab applies the same spreading geometry to electric and gravitational fields. Choose the spreading square bundle or the full radial field. Change the source charge or mass and compare the force on the same test object at two distances. Electric forces can attract or repel, while gravitational forces on positive masses point towards the source.
Follow a fixed bundle of field lines through two squares. Its area grows as r², while its mean normal field falls as 1/r². As a result, the flux through corresponding surfaces stays constant. On a flat square, the radial field is tilted towards the corners, so the calculation uses the normal component across the whole surface. Switch to curved spherical patches to see surfaces that are perpendicular to the radial field.
Model assumptions and suggested investigations
The simulation assumes ideal point sources in a transparent medium or vacuum. Distances are measured from the source centre. There is no absorption, scattering or background light. Field lines represent a static field; they are not moving particles. The moving light markers represent equal amounts of energy, so increasing power increases their emission rate.
Try doubling distance, doubling source power, reversing the test charge and doubling the test mass. Explain which changes affect field strength, force or flux. Then test your predictions using the numerical readouts and the Challenge tab.
Explore more free physics simulations, practise with A-Level Physics questions by topic, or visit Maths With Matt for free GCSE and A-Level resources and online tuition for students across the UK.
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Reference inspiration: The Physics Classroom and Martin Kirby’s Inverse Square Law Sim. This Maths With Matt version uses its own implementation, field models and flux calculations.