In the electromagnetic wave lengths say from x-rays to the 21 cm hydrogen line, the wave has a wave length. When it hits a receiver, eye, camera sensor whatever, it is turned into an electrical signal that can be interpreted by that device. Now we get to my question, is there an echo of that wavelength? Why I can hear you ask. If there is an echo then a generic receiver could be built that covered all wave lengths at the same time. I know the answer "no echo" but has anyone looked?