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MORE ON FLUX, LUMINOSITY AND THE INVERSE SQUARE LAW
We've defined what flux and luminosity mean, but there is one point where it can get a little confusing. If you want to measure the amount of energy that goes onto a particular object from a light source, you multiply the flux from that light source by the area of the object. The answer is in Watts, but it's not the luminosity of the light source; it's the luminosity incident on the object! Don't mix up these two different "luminosities." e.g. If you're looking at a candle flame and its flux on your eye is 0.01 W/m^2, and your dilated pupil has an area of about 0.000025 m^2, then the luminosity that gets into your eye is L = F * (area) = 0.01 W/m^2 * 0.000025 m^2 = 2.5 * 10^(-7) W. e.g. If that candle is one meter away from your eye, its luminosity is L = 0.01 W/m^2 * 4 * pi * (1 m)^2 = 0.126 W. REMEMBER THIS: to help keep the concepts of luminosity and flux clear, keep in mind that (1) the LUMINOSITY of an object doesn't change no matter how far away you are from it; luminosity is a property of the object. (2) the FLUX onto an object doesn't change no matter how big it is; flux is a property of distance from the luminous source. |