\documentclass[10pt, a4paper]{article} % twocolumn
\usepackage[utf8]{inputenc}
\usepackage[russian]{babel}
\usepackage[left=2cm, right=2cm, top=2cm, bottom=2cm]{geometry}
\usepackage{amsmath, amssymb}

    \usepackage{fancyhdr}
    \pagestyle{fancy}
    \fancyhf{}

      \renewcommand{\headrulewidth}{0.5pt}
      \renewcommand{\footrulewidth}{0pt}

      \lhead{\textsf{Оптика}}
      \rfoot{\thepage}

    % Equations Numeration.
      \numberwithin{equation}{section}

      % Formatting parameters.
      \setlength{\parskip}{6pt}
      \setlength{\parindent}{0pt}

      % Custom macrodefinitions.
      \newcommand{\avr}[1]{\left< #1 \right>}
      \newcommand{\dif}[2]{\cfrac{d#2}{d#1}}
      \newcommand{\pard}[2]{\cfrac{\partial#2}{\partial#1}}
      \newcommand{\pardd}[2]{\cfrac{\partial^2 #2}{\partial^2 #1}}
      \newcommand{\vect}[1]{\mathbf{#1}}
      \newcommand{\bs}{\boldsymbol}
      \newcommand{\norm}[1]{\left| #1 \right|}
      \newcommand{\wrap}[1]{#1\nobreak\discretionary{}{#1}{}}

      \newcommand{\subsc}[1]{\lefteqn{\scriptstyle#1}}
      \newcommand{\ointegrate}[1][]{\oint\limits_{\subsc{#1}}}
      \newcommand{\integrate}[2]{\int\limits_{\subsc{#1}}^{\subsc{#2}}}

      \newcommand{\Div}{\operatorname{div}}
      \newcommand{\Rot}{\operatorname{rot}}
      \newcommand{\Grad}{\operatorname{grad}}
      \newcommand{\nab}{\bs \nabla}
      \newcommand{\lap}{\Delta}

      \newcommand{\comment}[1]{\quad\text{#1}\quad}

      \renewcommand{\leq}{\leqslant}
      \renewcommand{\geq}{\geqslant}

      \renewcommand{\Re}{\operatorname{Re}}