Inverse square law
Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law.The inverse square law describes the intensity of light at different distances from a light source. Every light source is different, but the intensity changes in the same way. The intensity of light is inversely proportional to the square of the distance.This means that as the distance from a light source increases, the intensity of light is equal to a value multiplied by 1/d2,. The proportional symbol, , is used to 
show how these relate. The relationship between the intensity of light at different distances from the same light source can be found by dividing one from the other.
Proportional:
\[light\; intensity\; is\; propor\; to \frac{1}{distance\; squsred} \]
\[I\; \alpha \; \frac{1}{d^{2}} \]
I = light intensity (candela, W/m2)
means "is proportional to"
d = distance from a light source (m)
Intensity at different distances:
I1 = light intensity at distance 1
I2 = light intensity at distance 2
d1 = distance 1 from light source (m)
d2 = distance 2 from light source (m)
let value of d2=b and d1= a and I2 = x
