How can a grid act as a reflector?  Doesn’t make much sense when you think of it, but when we are talking about electromagnetic radiation, they definitely can.  Probably the most common example you see is in satellite TV offset reflector antennas, but you also see it in radio telescopes.  The dish is not made from a solid structure, it’s a mesh or has holes in it, and yet it can easily reflect the electromagnetic radiation to the receiver at the focal point of the parabaloid.  There are other structures that you can make that are not solid metal, but act as a radio frequency mirror.