Calculating Intensity of 13-Watt Bulb at 1.2m Distance

[looi76]

Homework Statement

A low energy, 13-watt bulb illuminates your desk from a distance of 1.2m. You can assume that it acts as a point source and that it is 100% efficient. What is the intensity (power per unit area) at the desk (in Wmˉ²)?

Homework Equations

Velocity = Frequency * Wavelength
Intensity is proportional to amplilude²

The Attempt at a Solution

I don't know how to start!

 

[Doc Al]

Imagine the bulb as a point source sending out its energy equally in all directions. By the time the light has traveled 1.2 m, over how large an area is the total power spread? Hint: What's the area of a sphere of that radius?