Why does a fluorescent tube act as a line source close up and a point source far away?

[kingkong]

hello,
iam doing an investigation on the inverse law using light. i want to know why can a flourescent tube act a line source? why is it that when you get too far the is acts a point source?

i look forward to your response

 

[gulsen]

inverse what law? inverse square?
your answer seems to be in calculus of the problem. try putting infinity for distance, and compare the result with point source's equation.