Inverse Square Law for Light
Posted: 12 Dec 2007, 09:04
Hi All (and Robb in particular!)
A little Physics problem.....
My Physics teacher was doing (last week) a prac investigating the inverse square law for light. Some of his students used a battery torch, others used microscope lamps as their light source. They measured light intensity with either an old "Lux Meter" or a data probe.
I suggested to him that he might not get very good results as his light sources were throwing the light out in only one direction. My thinking was that the inverse square law works because the light intensity depends on the number of "light beams" per unit area. As the distance from the source increases, the surface area of the square containing that many light beams, increases with the square of the distance, hence the intensity is the inverse (hence inverse square law!!) I reasoned that with a perfectly parallel source (eg a laser) the intensity would not drop off at all. I then reasoned that as you moved from one extreme to the other (eg point source radiating in all directions, to parallel beam) the inverse square law would hold less and less..
The class did the experiment and his students handed in a PERFECT STRAIGHT LINE graph of I against 1/r2 (!!!!!)
It made me feel like a bit of a dill! Can you explain where I went wrong, Robb?
I can see that I will be worrying about this for the whole of the Christmas holidays unless I have an answer!
Happy Christmas
Ian
A little Physics problem.....
My Physics teacher was doing (last week) a prac investigating the inverse square law for light. Some of his students used a battery torch, others used microscope lamps as their light source. They measured light intensity with either an old "Lux Meter" or a data probe.
I suggested to him that he might not get very good results as his light sources were throwing the light out in only one direction. My thinking was that the inverse square law works because the light intensity depends on the number of "light beams" per unit area. As the distance from the source increases, the surface area of the square containing that many light beams, increases with the square of the distance, hence the intensity is the inverse (hence inverse square law!!) I reasoned that with a perfectly parallel source (eg a laser) the intensity would not drop off at all. I then reasoned that as you moved from one extreme to the other (eg point source radiating in all directions, to parallel beam) the inverse square law would hold less and less..
The class did the experiment and his students handed in a PERFECT STRAIGHT LINE graph of I against 1/r2 (!!!!!)
It made me feel like a bit of a dill! Can you explain where I went wrong, Robb?
I can see that I will be worrying about this for the whole of the Christmas holidays unless I have an answer!
Happy Christmas
Ian