Intensity of Light

Question

Intensity of Light. The intensity I of light from a light bulb varies inversely as the square of the distance d from the bulb. Suppose that I is 90 W/m^2 (watts per square meter) when the distance is 5 m. How much farther would it be to a point where the intensity is 40 W/m^2

Answer 1

As we know: I = P/A , P is power of source (source can be a light or a sound). A is area which is 4πR²

and R is distance from the source.

I₁ = P/A₁ and I₂ = P/A₂ (Power of source is constant and intensity will change as distance changes).

P = I₁ . A₁ → P = (90).(4π.25) , A₂ = P/I₂ → (4πR²₂ ) = (90).(4π.25) / (40) → R²₂ = (90).(25) / (40)

R₂ = (3).(5) / (2) → R₂ = 7.5 m