Another SAT II Physics question: optics

[SOLVED] another SAT II Physics question: optics

Homework Statement

A point source light which has an intensity level of 10 candle power at a distance of 2 m will have an intensity at 4 m of
A) .5 cp
B) 1 cp
C) 2.5 cp
D) 5 cp
E) 10 cp

Homework Equations

i don't really know...my book doesn't really address this
i don't even know what a point source light does.

The Attempt at a Solution

i tried to set up a proportion, but i got 20 as my answer, which is obviously wrong!

anything to start me off would be greatly appreciated!

You need to know the relationship between distance and intensity.

It is basically,

$I \propto \frac{1}{d^2}$

With that, you should be able to solve your problem.

oh okay;;
so since the distance was doubled, the intensity would decrease by a factor of 1/4,
and 10 x (1/4) = 2.5 cp
yes??

yes.

oh okay;;
so since the distance was doubled, the intensity would decrease by a factor of 1/4,
and 10 x (1/4) = 2.5 cp
yes??
Exactly.