# Momentum density vector of a plane em wave

## Homework Statement

Show that in vacuum the magnitude of the momentum density
vector of a plane electromagnetic wave is equal to the energy density
divided by the speed of light, c.

## Homework Equations

$\vec{S}$ = $\vec{E}$x$\vec{H}$

## The Attempt at a Solution

This doesn't seem too hard if you know what a momentum density vector is, so if this should be in the introductory section I apologize but it is a hw question from an upper division course.

The only thing I'm really looking for is how to find/ what is a momentum density vector. I have no idea how to go about finding it.