How to stop a heat ray gun

See http://en.wikipedia.org/wiki/Active_Denial_System for a description of the weapon.

It's just EM radiation so kitchen foil would probably shield you just fine. Or soak your clothes in salt water. Anything conductive would do.

I have no knowledge beyond general physics expertise. It strikes me that all weapons are a recourse to brutality when they are used by one human against others. This weapon is basically intended to cook people while they are still alive, or possibly blind them, by the same principle as meat is cooked in a microwave oven. I hesitate to be so blunt about it but I prefer that bluntness to euphemisms such as "deployed" and "operational evaluation".

Microwaves are waves of electromagnetic radiation. To protect a body from them one must either reflect them or introduce a barrier that absorbs them. An ordinary mirror will work quite well as a reflector, or else large amounts of aluminium foil or things like that. However, just because something is reflective in the visible spectrum it does not follow that it will be reflective for microwaves. One can do experiments with any ordinary microwave oven to discover what works. Just put a cup of water in the oven and time how long it takes to heat up by some given amount. Then wrap it in your test material and try again. However there will be safety issues and possible sparks so I would not recommend doing this before you have informed yourself a bit further about the safety issues.

To absorb microwaves intended to heat humans you can put in the way anything which interacts with microwaves similarly to the human body, and the most obvious thing to try is water. Anything with a high water content will act as a barrier, but the difficulty now is to know how thick a barrier you will need, or how quickly it will heat up. Since water also has a high heat capacity it will heat up more slowly than most things, which is good, but it will also evaporate and of course it is heavy to carry around.

If one had time to plan and organize then it is worth noting that, unlike bullets, microwaves can be reflected without losing much of their power, and one could in principle reflect them back. It might make someone think twice about using such a weapon if they faced a serious risk of having it reflected back at them. But ultimately the only strategy with a long-term hope of success is to talk to the enemy and try to discover what they are afraid of and what has caused their descent into brutal treatment of others.