# Illuminance and distance

Hello,

We had a lab in physics about the relationship between illuminance and distance. Our teacher gave use data so we make a graph of them. E is the illuminance in Lux and d is the distance in cm. (d,E) (50cm, 94 lux) (60cm, 64 lux) (70 cm, 49 lux) (80cm, 40 lux) (90 cm, 32 lux) (100cm, 26 lux)
It the graph, when I connect the points it give me an inverse proportion. My objective is to find an equation.

What I first tried is to find an equation directly from this graph but after searching on the internet I found this formula : E=I/d^2 and this explanation : intensity is inversely proportional to the square of the distance from the source. I do not understand, in my graph d is my abscissa not d^2 . I do not understand how we can use d^2 instead of d.

Last edited:

CWatters
Homework Helper
Gold Member
Are you familiar with the equation for a straight line? Compare the standard equation for a straight line...

y = mx + c

with your equation which I have rewritten slightly..

E = I(1/d2) + 0

Then replot your data suitably modified so you get a straight line. Come back if you get stuck.

PS: It's not d2 you use.

Are you familiar with the equation for a straight line? Compare the standard equation for a straight line...

y = mx + c

with your equation which I have rewritten slightly..

E = I(1/d2) + 0

Then replot your data suitably modified so you get a straight line. Come back if you get stuck.

PS: It's not d2 you use.

E = I(1/d2) + 0

my question why is the equation not E=I(1/d)

where do the 2 come from

CWatters