Question

PART A: A krypton laser produces a cylindrical red laser beam 2.0 mm in diameter with 2.0 W of power. What is the light intensity on a wall 0.60 m away from the laser?

PART B: A typical 100 W lightbulb produces 4.0 W of visible light. (The other 96 W are dissipated as heat and infrared radiation.) What is the light intensity on a wall 0.60 m away from the lightbulb?

Answer 1

A)

the area is just the cross-sectional area of the beam, because the laser is a uni-directional light source.
So area A = *r², and Intensity = Power / Area

radius r = d/2 = 2.0*10^-3m / 2 = 10^-3 m

I = 2.0 W / *0.001² = 6.366*10⁵ w/m² at any distance, including 0.6 m away.

part B:

Intensity I = P/A
When light is spread in all directions A is the surface area of a sphere

For a sphere (lightbulb), the area is 4π*r².

==> I = P/4πr^2 = 4.0 / 4π*0.60^2 = 0.884 W/m^2