5.1 INTRODUCTION
The light from a fire or flame will propagate spherically outwards from the source. In general, the apparent intensity of the light will vary inversely with the distance from the observer to the source. The earliest unit for measuring light, the foot-candle, was based on the amount of light being emitted from a single candle at a distance of one foot. The more candles lit, the brighter the radiant light.
Since light waves propagate spherically from a candle, the number of photons per unit area will vary based on the distance to the candle. The measurement of light intensity is called luminous flux.
Experimental ...
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