Originally Posted by

**AlvinCY**

Question 3: The intensity of light as measured from a light bulb varies inversely as the square of the distance "d" from the bulb. Suppose that $\displaystyle I = 90 W/m^2 $ when the distance is 5 m. What would the light intensity be 10m from the bulb?

We know that I varies inversely to the square of the distance, d. In another words:

$\displaystyle I = k \times \frac{1}{d^2}$ for some constant k.

We can work k out by substituting $\displaystyle I = 90, d = 5$

$\displaystyle 90 = k \times \frac{1}{25}$

$\displaystyle k = 2250$, therefore,

$\displaystyle I = 2250 \times \frac{1}{d^2}$

For d = 10, $\displaystyle I = 2250 \times \frac{1}{100} = 22.5 W/m^2$