Topic Menu

Cyber-chess a chess instruction site

Intensity

Progressive waves transfer energy away from the source.

Power (P) is a measure of how much energy is transferred in unit time and

Intensity (I) is the power per unit area (A).

intensity = power/area

I = P/A

The power will spread out in all directions - radially. This means that at distance r away from the source, the light energy is spread out over a sphere with radius r, so:

I = P/(4πr2)

The amplitude will also change as the wave spreads out.

The energy of a wave is proportional to its amplitude squared, so intensity is also:

I ∝ amplitude2

Example Question:

A light bulb has a power output of 5.0 W, what will the intensity of the light be at a distance of 1.5 m from the filament?

P = 5.0 W

r = 1.5 m

I = P/(4πr2)

I = 5.0/(4π x 1.52)

0.18 W m-2