Question

A spherically-spreading EM wave comes from a 104.0 W source. At a distance of 9.6 m, what is the intensity of the wave?

Answer 1

Approximately 0.0898 W/m².

Step-by-step explanation:

The intensity of light measures the power that the light delivers per unit area.

The source in this question delivers a constant power of
`\rm 104.0\; W`. If the source here is a point source, that
`\rm 104.0\; W` of power will be spread out evenly over a spherical surface that is centered at the point source. In this case, the radius of the surface will be 9.6 meters.

The surface area of a sphere of radius
`r` is equal to
`4\pi r^(2)`. For the imaginary 9.6-meter sphere here, the surface area will be:

`\rm 4\pi * (9.6\; m)^(2) \approx 1158.12\; m^(2)`.

That
`\rm 104.0\; W` power is spread out evenly over this 9.6-meter sphere. The power delivered per unit area will be:

`\displaystyle\rm  (104.0\; W)/(1158.12\; m^(2))\approx 0.0898\; W\cdot m^(-2)`.