Why does the efficiency decrease as the distant increases?

[VCE_2012]

Hey guys I am doing an analysis on an experiment I have completed regarding Photovoltaic cells, and my question is- Why does the efficiency of energy conversion decrease as the distant of the light source (being a lamp) increases?

Additional info: the radius of the light is noted to increase as the distance increases, shouldn't that increase the efficiency as more area on the solar panel is exposed to light OR the concentration is decreased as light spreads.

I just want a general answer...

[moekamo]

intensity of light follows an inverse square law, so, for example, by doubling the distance, the intensity decreases by a factor of 4. i don't know how solar panels work, but i'd guess that their efficiency must somehow be dependent on the intensity of light incident on it, rather than the 'amount' of light, if that can even be measured...

Also it depends how big the panel is, you will eventually get to a point when the light spreads past the panel and so now not all the light shines on the panel, obviously this will decrease the intensity since the light is more spread out as well as shining on other surfaces that aren't the solar panel.

anyway hope this helps...somehow i don't think it will :/