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## 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

[itex]\vec{S}[/itex] = [itex]\vec{E}[/itex]x[itex]\vec{H}[/itex]

## 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.