... Before the modern era of technological astronomy, to know anything about the Universe beyond our planet we relied on light. ... Historically the two rival ideas were that light was either a stream of tiny bullet-like particles or a wave wobbling its way through space. ... The light we see (visible light which is not the tautology it sounds) is only part of the electromagnetic spectrum, a great sweep of radiations which also includes radio, infra-red, ultra-violet, x-rays and gamma rays. ...