How Far Should the 70-cd Lamp Be to Match the 10-cd Lamp's Intensity?

[bnwchbammer]

Homework Statement

A 10-cd point source lamp and a 70-cd point source lamp cast equal intensities on a wall. If the 10-cd lamp is 8.0 m from the wall, how far from the wall is the 70-cd lamp?

Homework Equations

I'm not sure what the equation for a candela is.

The Attempt at a Solution

Can't attempt it without knowing an equation.


That's the only information given, and I don't know how to form an equation.

 

[nickjer]

A candela is a unit of intensity for light. And you remember that intensity falls off as 1/r^2.