Physics masters with undergraduate in computer science
I would say you would absolutely be able to get in to a physics masters program as long as you have taken the pre-requisites for the specific program.  Most programs have a list of courses that must be taken to be considered for admission.

I got my undergraduate degree in Geology and Geophysics, but I became more interested in engineering towards the later part of my undergraduate.  So I was sure to take all of the requisite engineering courses that I needed to get into an engineering masters program.  Now I'm actually working on a PhD in Civil Engineering.  

Moral of the story... It is absolutely possible, but be sure to check out the program requirements for the physics programs you are interested in applying.