Calculating Average Intensity of a laser blackboard pointer

[patelpalak]

Homework Statement

A laser blackboard pointer delevers 0.10-mW average power in a beam 0.90 mm in diameter.

Homework Equations

a) Find the average intensity.
b) Find the peak electric field.
c) Find the peak magnetic field.

The Attempt at a Solution

ok for part a) i used the formula ave. s = p/4 pie r^2 (as p=.10 mW and r=0.45 mm)
but i didnt get the right answer
so am i using the wrong formula or the wrong value for r?


for part (b) and (c) i can't do anythin bout it because i need the average intensity
in order to find the elecric and magnetic fields.

somebody plez help me asap...thanx!

 

[LowlyPion]

Isn't the power measured in the area of the beam?

Avg power = .1*10^-3w / (3.14*(.45*10^-3m)²) = ... w/m²

 

[patelpalak]

Isn't the power measured in the area of the beam?

Avg power = .1*10^-3w / (3.14*(.45*10^-3m)²) = ... w/m²

is there 4 in the bottoom becuz isn't the
formula is ave s= p/4*pie*r^2 ?

 

[LowlyPion]

is there 4 in the bottoom becuz isn't the
formula is ave s= p/4*pie*r^2 ?

That r is for distance isn't it?

This is a laser beam and I'd say dispersion is minimal, so you only want the cross section of the beam.

 

[patelpalak]

ok i got 160 W/m^2
thanx for the help!

 

[patelpalak]

hey i got part b
but have problem with part C

is this right?

B^2 = ave. S *2* 4*pie*10^-7/ (3*10^8) = T