Average intensity problem? related to EM waves?

[waterbottle12]

Homework Statement

The helium-neon lasers that we use in lab produce light waves with an average power
of 0.85 mW. Suppose that at a distance of 1.0 m from the laser the beam has a diameter
of 1.1 mm. a) What is the average intensity of the laser beam 1.0 m from the laser? b)
What is the maximum electric field 1.0 m from the laser?

Homework Equations

I = Energy/ (area x time)

I= 0.5 (circly E with subscript o) x c x E^2

The Attempt at a Solution

a. I think its just power divided by area with area being pi times (1.1mm/2)^2
because Power is just energy over time. just double checking
b. when I say circly E thing I mean permitivity of free space (I'm sure that's spelled wrong)
and I just plug into solve for E right?...but than why did the question give "distance of 1.0 m from the laser"...is it just like superfluous info?

 

[Redbelly98]

Welcome to Physics Forums!

The Attempt at a Solution

a. I think its just power divided by area with area being pi times (1.1mm/2)^2
because Power is just energy over time. just double checking
b. when I say circly E thing I mean permitivity of free space (I'm sure that's spelled wrong)
and I just plug into solve for E right?...but than why did the question give "distance of 1.0 m from the laser"...is it just like superfluous info?

Yes, you are entirely correct for both parts.

For (b), they tell you it's at 1.0 m from the laser because you know what the intensity is at that distance. At other distances, the intensity can be something else (because the beam diameter can be different) and therefore you can't figure out what E is.

p.s. To type ε_o you can copy-and-paste this: [noparse]ε_o[/noparse]