Inverse Square Law for Light

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Ian
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Inverse Square Law for Light

Post by Ian »

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
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Robb
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Re: Inverse Square Law for Light

Post by Robb »

HI Ian,

This is quite a problem you have faced...

Firstly, I like the way that you had presented your thoughts, The light source you use (eg, Lamp on a stand) is suffice for this prac. What you need to think about, (and I hope I can explain it here for you without having a lamp to demonstrate), is that a lamp emits in a 3D area, yes? Now, the same intensity and the same amount of photons are striking an object, (eg my hand, if I hold it up close to it), no matter where in the 3D area it place my hand. That is why it should be fine to hypothesise the use of the Normal Light On a Stand..

Your Laser prac seemed to be O.K in theory but the problem had been the focus point of only a small amount of light across the whole surface area of the Lux Meter detector. With that in mind I can see how there would be consistent Straight lines when graphing the Inv. Sq. (Here is a Question for you.. As you use a Laser what will happen is that the intensity will actually increase rather than decrease as the Laser is moved further away from the meter??? Why?) I will tell you later if you can't work it out.. Keep in mind though as the laser is moved away the beam does not spread over a large area of the detector or grid paper for compasison.

But primarily to get back to your problem... The amount of light that you have coming from a 3D light source eg. Lamp on a stand, will increase in area as the distance increases. Therefore if you are looking at a particular spot on a piece of paper of the light meter then there is more light upon that but with a drop in intensity.

That should give reasonable results.. and what should be observed is a scatter graph but a linear one at that.

This should help you out heaps...

Cheers,

Robb.....
Last edited by Robb on 15 Dec 2007, 22:17, edited 3 times in total.
Reason: Edited Post
Dr Robert Crosdale. MRACI. NSS. NSSA. NASA.
Ph.D (Chem), Post Grad Ph.D (Physics), M.Ed, B.Sc (Hons), Dip. Appl. Sc. (Chem)
Lake Munmorah High School.
University of New England.
University of New South Wales.
University of Newcastle.

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PhysicsOne
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Re: Inverse Square Law for Light

Post by PhysicsOne »

For this prac. you can use an axial filament light globe. So long as the filament of the light globe looks like a "point" when you stare at it, it should act like a point source of light (no reflector).
I made the mistake of using a light globe that gave me a rectangular illuminated area. The filament was in the wrong direction.So the inverse square law didn't hold. I worked out later my mistake.
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