You could try shadings:
\documentclass[tikz,border=5]{standalone}
\begin{document}
\tikz[x=0.125cm,y=0.125cm]
\foreach \i in {0,1,...,21}
\path [left color=black, right color=white, shading angle={mod(\i,20)*180+90}]
(\i*.9,0) rectangle ++(1,10);
\end{document}

Or fadings:
\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{fadings}
\begin{tikzfadingfrompicture}[name=interference]
\foreach \i in {0,1,...,21}
\path [left color=transparent!0, right color=transparent!100, shading angle={mod(\i,2)*180+90}]
(\i*.9,0) rectangle ++(1,20);
\end{tikzfadingfrompicture}
\begin{document}
\foreach \i in {0,...,24}{
\begin{tikzpicture}
\clip (-5,-5) rectangle ++(10,10);
\path [fill=black, path fading=interference, fit fading=false, fading transform={rotate=\i*7.2}] (-10,-5]) rectangle ++(20,10);
\path [fill=black, path fading=interference, fit fading=false] (-10,-5) rectangle ++(20,10);
\end{tikzpicture}
}
\end{document}

In both cases using \i*.9
is to workaround some annoying white lines that appear between adjacent shadings which may (or may not) be viewer artifacts.
To change the phase of the interference pattern is tricky to do with shading but with fadings it is fairly easy if the fading is specified over a larger area than the required path. Then the fading transform
key can be used to shift the fading that is "seen through" the path.
In the following example the red rectangle illustrates the position of the fading relative to the area that is faded. Also, a line is drawn to fill in the white lines between adjacent shadings instead of the workaround described above to get rid of the white lines that appear between adjacent shadings.
\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{fadings}
\begin{tikzfadingfrompicture}[name=interference]
\foreach \i in {0,1,...,19}{
\path [left color=transparent!0, right color=transparent!100, shading angle={mod(\i,2)*180+90}]
(\i,0) rectangle ++(1,10);
\ifodd\i\else% Fill in gap
\path [draw=transparent!0] (\i,0) -- ++(0,10);
\fi
}
\end{tikzfadingfrompicture}
\begin{document}
\foreach \i in {-10,...,10,9,8,...,-9}{%
\begin{tikzpicture}
\path [draw=red, shift=(0:\i/2)] (-10,-5) rectangle (10,5);
\path [fill=black, path fading=interference, fit fading=false, fading transform={shift=(0:\i/2)}] (-5,-5) rectangle (5,5);
\useasboundingbox (-15,-5) rectangle (15,5);
\end{tikzpicture}%
}
\end{document}

Using fadings means you can also do cool stuff like this:
\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{fadings}
\begin{tikzfadingfrompicture}[name=interference]
\foreach \i in {1,...,15}
\foreach \j in {1,...,25}
\path [line width=\j, draw=transparent!0,opacity=1/30]
(0:\i) arc (0:180:\i);
\end{tikzfadingfrompicture}
\begin{document}
\begin{tikzpicture}
\clip (-5,-5) rectangle ++(10,10);
\path [fill=black, path fading=interference, fit fading=false] (-10,-5) rectangle ++(20,10);
\path [fill=black, path fading=interference, fit fading=false, fading transform={rotate=45}] (-10,-5) rectangle ++(20,10);
\end{tikzpicture}
\end{document}

\documentclass{article} \usepackage{tikz} \usetikzlibrary{shadings} \begin{document} \begin{tikzpicture} \shade[left color=gray!0,right color=gray!100] (0,0) rectangle (2,1); \shade[left color=gray!0,right color=gray!100] (2,0) rectangle (3,1); \shade[left color=gray!0,right color=gray!100] (3,0) rectangle (6,1); \shade[left color=gray!0,right color=gray!100] (6,0) rectangle (7,1); \end{tikzpicture} \end{document}