Question

A. 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 2.0 m away from the lightbulb?

b. 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 2.0 m away from the laser?

Answer 1

a) 0.08 W/m² b) 0.64*10⁶ W/m²

Step-by-step explanation:

a) Assuming that the lightbulb behaves as a point source, it radiates energy evenly in all directions, so the power spreads out over the surface of a sphere of radius equal to the distance to the object being illuminated.

The expression for intensity, in this particular case, is as follows:

`I =(P)/(A) (W/m2) = (4.0 W)/(4*\pi*(2.0m)^(2) ) = 0.08 W/m2`

b) In the case of light produced by a laser source, the beam is not spread out throughout space, so we can simply define the intensity as power divided by the area of the beam, assumed circular, as follows:

`I =(P)/(A) (W/m2) = (2.0 W)/(\pi*(0.001m)^(2) ) = 0.64e6 W/m2`

⇒ I = 0.64*10⁶ W/m²