How Does Doubling the Distance Affect Light Intensity?

[oceanflavored]

[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!

PLEASE HELP!
anything to start me off would be greatly appreciated!

 

[Erythro73]

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.

 

[oceanflavored]

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??

 

[spideyunlimit]

yes.

 

[Erythro73]

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.