List variables in equation

Question

My code looks horrendous

When monochromatic light is shone through a solution of iron, Beer's law states that 

$$ A = \varepsilon l c $$
\begin{tabular}{l}
      Where $A$ is amount of light absorbed \\
      \quad \quad  \quad $\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
      \quad \quad  \quad but not with concentration\\
      \quad  \quad \quad $l$ is the thickness of the sample

\end{tabular}

Can it be improved?

Answer 1

Please have a look on Why is \[ ... \] preferable to $$ ... $$?, the description of the variables can be done in description or in an itemize list, for example.

From a personal point of view, I would rather use \ell instead of l as equation identifier.

\documentclass{article}

\usepackage{mathtools}

\begin{document}
When monochromatic light is shone through a solution of iron, Beer's law states that 

\[ A = \varepsilon l c \]

\noindent where
\begin{description}
\item  $A$ is amount of light absorbed 
\item  $\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
  but not with concentration
\item  $l$ is the thickness of the sample
\end{description}


\end{document}

Another (better?) solution, with more fine control of the list parameters, better optical appealance, but this is a matter of taste actually.

\documentclass{article}

\usepackage{enumitem}
\usepackage{mathtools}

\begin{document}
When monochromatic light is shone through a solution of iron, Beer's law states that 

\[ A = \varepsilon \ell c \]

\noindent where
\begin{description}
\item $A$   is amount of light absorbed 
\item  $\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
  but not with concentration
\item  $\ell$ is the thickness of the sample
\end{description}


\noindent where
\begin{description}[labelindent=10pt,labelsep=10pt]
\item[$A$]   is amount of light absorbed 
\item[$\varepsilon$] is the molar absorbance coefficient, which varies with wavelength,\\
  but not with concentration
\item[$\ell$] is the thickness of the sample
\end{description}

\end{document}

Answer 2

Another alternative is to use a (d)cases environment:

Here's the code:

\documentclass{article}
\usepackage{mathtools}
\begin{document}

  When monochromatic light is shone through a solution of iron, Beer's law states that
  \[ A = \varepsilon l c ,\quad
    \text{where }\quad
    \begin{dcases*}
        A& is amount of light absorbed \\
        \varepsilon& is the molar absorbance coefficient, which varies \\
                   & with wavelength but not with concentration\\
        l& is the thickness of the sample
  \end{dcases*}
  \]

\end{document}

Of course, you could ust use an ordinary cases environment but I prefer dcases from the mathtools package.

Answer 3

Two tricky examples (your code at the beginning):

\documentclass{article}
\begin{document}

When monochromatic light is shone through a solution of iron, Beer's law states that 

$$ A = \varepsilon l c $$
\begin{tabular}{l}
      Where $A$ is amount of light absorbed \\
      \quad \quad  \quad $\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
      \quad \quad  \quad but not with concentration\\
      \quad  \quad \quad $l$ is the thickness of the sample
\end{tabular}

\bigskip

When monochromatic light is shone through a solution of iron, Beer's law states that 
\[
 A = \varepsilon l c 
\]
 Where $A$ is amount of light absorbed 

\noindent\hangindent3em\hangafter0
      $\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
     but not with concentration\\
       $l$ is the thickness of the sample



When monochromatic light is shone through a solution of iron, Beer's law states that 
\[
 A = \varepsilon l c 
\]
Where $A$ is amount of light absorbed \\
 \hspace*{3em}%     
\begin{tabular}{l}
$\varepsilon$ is the molar absorbance coefficient, which varies with wavelength,\\
       but not with concentration\\
       $l$ is the thickness of the sample
\end{tabular}

\end{document}

Answer 4

Here's another solution, which uses an align* environment. Its virtue, so to speak, is that the left-hand edges of the main equation and of the description lines are aligned. Two reasons for using a \parbox for the description of \varepsilon: (i) no need to select a line break by hand, and (ii) the interline spacing inside the parbox won't be as wide as across the other lines of the align* environment. The only choice that needs to be made is the width of the parbox -- I happened to select 3in, but you should feel free to adjust this to suit your needs.

\documentclass{article}
\usepackage{mathtools,ragged2e}

\begin{document}
When monochromatic light is shone through a solution of iron, Beer's law states that 
\begin{align*}
A &= \varepsilon l c
\shortintertext{where}
A &\text{ is the amount of light absorbed,}\\
\varepsilon &\text{ \parbox[t]{3in}{\RaggedRight is the molar absorbance coefficient, which 
   varies with wavelength but not with concentration, and}}\\
l &\text{ is the thickness of the sample.}
\end{align*}

\end{document}

Answer 5

I'm not sure if this helps but this is how usually I do what yo asks:

\documentclass{article}    
\usepackage{amsmath}    

\begin{document}
When monochromatic light is shone through a solution of iron, Beer's law states that:

\begin{equation*} % The starred version avoid the automatic number.
 A = \varepsilon l c
 \end{equation*} 

\noindent Where:\\ 
$A$ is amount of light absorbed, \\
$\varepsilon$ is the molar absorbance coefficient, which varies with wavelength, but not with concentration, and\\
$l$ is the thickness of the sample.
\end{document}

I use amsmathfor use the starred version of equation environment that produces the same than equation but the number of the equation. I don't use the tabular environment, and the alignment is very simple. It works for you?