Psi is different in beamer and usual tex documents

Question

I don't know why, but the \Psi char displayed in beamer is different from the Psi in usual tex files:

what I want/have in classic tex files:

What I have in Beamer:

Thank you!

MWE:

\documentclass[]{beamer}
\begin{document}
\begin{frame}{Title}
  $\Psi$
\end{frame}
\end{document}

Answer 1

If you want to use this few times, you can do $\mathrm{\Psi}$. But, if you want to use serif fonts in whole document, use

\usefonttheme{serif}

As pointed out by Medina in this answer you can use serif fonts only in math mode using the option

\usefonttheme[onlymath]{serif}