The predicted relationship between brightness and distance is called the inverse square law. It says that when the distance from a light doubles, its brightness should decrease by a factor of four. The equation for the brightness, written as I in the equation below, and the distance from the light, written as d, is
In this equation, A is a constant that depends on how luminous the light is (in other words, what "wattage" the light bulb is). Also, the notation d2 is short hand for d X d ("d times d").
You do not need to understand this equation in detail. The important point is that the brightness depends on distance, and that when the distance doubles, the brightness goes down by a factor of four. This relationship is used in the table on your worksheet in which you calculate the predictions that the inverse square law makes.