# The Naked Scientists Forum

### Author Topic: Is this observation knew to science?  (Read 1431 times)

#### sciconoclast

• Sr. Member
• Posts: 149
##### Is this observation knew to science?
« on: 31/01/2011 20:29:21 »
I have noticed that the ability of light from a second polarized filter to pass through a differently oriented third filter depends in part on the orientation of the first, or prior, filter.   Up to this point I was under the impression that, as per quantum theory, only the cosine of the angle between the second and third filter orientations determines the percentage of light passing through the third filter.   What I have seen must have been observed by others but I can not find anything describing this affect.

An example would be: if filter #1 is at 90 deg., and filter #2 is at 45 deg., substantially more light can pas through filter #3 if it is at 90 deg. rather than 0 deg.   This effect can be multiplied if #1 is 90 deg., #2 is 45 deg., #3 is 90 deg., #4 is 45 deg..   Then the ratio of light passing through #5 at 90 deg. to #5 at 0 deg. is even greater.   This can be reversed if #3 is changed to 0 deg. but the total amount of light passing in this set will be much less than the previous one.

If this turns out to be something new I will be back with more details.   However, I expect that someone will inform me that this is the such and such affect and here is the formula for calculating it.  But sense I couldn't find it on my own I felt compelled to ask.

#### Bored chemist

• Neilep Level Member
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##### Is this observation knew to science?
« Reply #1 on: 31/01/2011 21:01:10 »
It is certainly well known that a third polarising filter placed between two crossed polarisers will increase the amount of light that gets throuigh.

#### JP

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##### Is this observation knew to science?
« Reply #2 on: 31/01/2011 21:07:35 »
I'm a bit confused by your description, but what I take home is that the amount of light passing through a 90-45-90 set of polarizers is different from that passing through a 90-45-0.  In fact, given perfect polarizers, the amount of light passing through these two setups should be identical.  This has been tested many times, so if you've found some deviation from this it's probably an error in your setup, not in the theory.  However, having used polarizers before, I can tell you that it can be very difficult to get them to work properly to a high degree of precision.  Without very careful alignment and high quality polarizers, you can easily get anomalous results.

#### sciconoclast

• Sr. Member
• Posts: 149
##### Is this observation knew to science?
« Reply #3 on: 31/01/2011 23:56:17 »
Thanks JP!

You understand the observation correctly.   Your are correct that I am not using high quality filters.   When I got the anomalous results I shifted the filters around and tried twenty two different alignment combinations to compensate for this and received consistent results.   However, I accept your comment that it has been checked many times and that you are probably correct that my set up was too tertiary.

Thanks for the information.

#### sciconoclast

• Sr. Member
• Posts: 149
##### Is this observation knew to science?
« Reply #4 on: 01/02/2011 15:30:30 »
Oh!!

I just realized that if a filter does not polarize light a hundred percent then the unpolarized light will retain the prior polarization and create a bias in the amount of light towards that same future polarization.   So I have learned a simple way to check the quality of a filter, not to try and save money on polarizers, and not to come to conclusions without considering all of the possibilities.

I was actually setting up for a different experiment when I ran amuck.   I will be using better polarizers when I finally get to that experiment.