Light Intensity Calculator Formula
Understand the math behind the light intensity calculator. Each variable explained with a worked example.
Formulas Used
Intensity
intensity = power / (4 * pi * pow(distance, 2))Variables
| Variable | Description | Default |
|---|---|---|
power | Light Source Power(W) | 100 |
distance | Distance(m) | 2 |
How It Works
Light Intensity (Inverse Square Law)
Light intensity from a point source decreases with the square of the distance.
Formula
I = P / (4 * pi * r²)
where P is the total power output and r is the distance from the source. Doubling the distance reduces intensity by a factor of four.
Worked Example
A 100 W light bulb at 2 meters distance.
- 01I = P / (4*pi*r²)
- 02I = 100 / (4*pi*4)
- 03I = 100 / 50.27
- 04I = 1.989 W/m²
Frequently Asked Questions
Why does the inverse square law apply?
A point source radiates uniformly in all directions. The same power spreads over an ever-increasing spherical area (4*pi*r²), reducing intensity.
What is the intensity of sunlight at Earth?
About 1361 W/m² above the atmosphere (the solar constant). At the surface, roughly 1000 W/m² on a clear day.
Does this apply to laser beams?
Not directly. Lasers produce a collimated beam that spreads slowly (governed by diffraction), so intensity drops much more slowly than 1/r².
Ready to run the numbers?
Open Light Intensity Calculator