[Date Index]
[Thread Index]
[Author Index]
Re: Re: Complex Oddity
*To*: mathgroup at smc.vnet.net
*Subject*: [mg57651] Re: [mg57631] Re: Complex Oddity
*From*: Pratik Desai <pdesai1 at umbc.edu>
*Date*: Fri, 3 Jun 2005 05:33:34 -0400 (EDT)
*References*: <d79enu$lbl$1@smc.vnet.net><d7hahj$3q6$1@smc.vnet.net> <200506020917.FAA11965@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
jfeth at azlink.com wrote:
>I use Jones matrices to evaluate optical circuits and always need to
>find the intensity of two interfering electric fields. The intensity
>(or brightness) of a field is found as a positive real value from a
>complex field as the field times the complex conjugate of the field.
>As you can see below, Mathematica gives yet another complex value when
>one does this straightforward multiplication instead of the real value
>that, I might add, is drilled into all students at the first glimpse of
> root(-1). For several weeks I struggled to use Mathematica in my
>circuit evaluation until one inspired Saturday, after several pots of
>coffee, dumb luck and iteration brought forth Intensity[expr_]:= below.
> I don't know why it works, I don't know how it works, and I
>don't know another way to do the job, but, even as ugly as it is, at
>least it works and it works very quickly. Importantly, it also gives
>me answers as cosines with arguments that are (real)sums and
>differences of characteristic delays, misalignments, and phase
>modulation terms.
>
>f=E^(I*d)
>
>In[1]:=
>f*Conjugate[f]
>
>Out[1]=
> E^ (I*d - I*Conjugate[d])
>
>In[2]:=
>Intensity[expr_]:=
>TrigReduce[ExpToTrig[expr*TrigToExp[ComplexExpand[Conjugate[ExpToTrig[expr]]]]]]
>
>In[3]:=
>Intensity[f]
>
>Out[3]=
>1
>
>With this solution in hand, as an optical engineer, I now have the
>luxury of wondering 1) exactly what mathematical elegance (or utility)
>is gained by Mathematica's assumption that every variable is always
>complex, and 2) why there is apparently no way in Mathematica to
>globally define a variable as a real number (i.e., its own conjugate).
>
>Regards,
>
>John Feth
>
>
>John Reed wrote:
>
>
>>Thanks to all who explained what is happening and how to work this problem
>>correctly. Now I know one facet of working with complex numbers. I don't
>>feel much better about this however. I received one e-mail that said this
>>was my fault for not reading the documentation closely enough. This problem
>>came up in the book "Mathematica for Physics" second edition by Zimmerman
>>and Olness. They solve a problem using the Complex[a_,b_]->a rule, ( see
>>page 91) but not the b part. The b part was my idea. Now I know why they
>>didn't solve for the imaginary part this way. They get the imaginary part
>>by subtracting the real part from the complex expression and dividing by I.
>>How many other gotchas are hidden in the code, waiting to bite the unwary
>>and relatively new user? What documentation tells about this kind of a
>>problem or do I just have to find them for myself by hopefully catching the
>>errors as they occur?
>>
>>John Reed
>>
>>
>>
>
>
>
I have also struggled with the very same questions, I have found the
TagSet defination invaluable here is how I would deal with your problem
TagSet[d, Im[d], 0];
TagSet[d, Re[d], d];
TagSet[d, Conjugate[d], d];
f[d_] := E^(I*d)
f[d]*Conjugate[f[d]]
>>1
The fundamental question remains why does Mathematica treat all
variables as complex, well the simple answer may be WHY NOT!!. You can
always tell mathematica which variables are real/complex, by giving them
thier identity(shown above) which is the way it is supposed to work,
isn't it? :-)
Best Regards
Pratik Desai
--
Pratik Desai
Graduate Student
UMBC
Department of Mechanical Engineering
Phone: 410 455 8134
Prev by Date:
**NonlinearFit**
Next by Date:
**Re: mml files**
Previous by thread:
**Re: Re: Complex Oddity**
Next by thread:
**Re: Re: Complex Oddity**
| |