The most striking result so far is that the spectrum of a light-beam can change as the light travels in free space.  Most people find that surprising. But in fact, experiments have been known for a long time now  -  I did one as an undergraduate which could have been interpreted in these terms, but wasn't.

Let me describe this experiment              

explanation to be added

This perhaps seems a pretty trivial experiment  -  or at any rate, it's one whose explanation doesn't require any recondite ideas.  Fine.  That's what I want you to believe  -  as light propagates its spectrum may change, for very simple reasons expressible in terms of the superposition of waves.

In March of this year Emil Wolf published a short paper in Physics Letters (Mar 31, 1986) which showed that

(i) in general, the spectrum of light can be expected to change as it propagates, so that the spectrum of the light-beam in the far-field region will not in general be the same as that of the source;
(ii) In order that the far-field spectrum of the light be the same as that of the source, the 'degree of spectral coherence' of the source (i.e., in effect, the mutual intensity determined from narrow-band filters) m(P₁, P₂; w) must depend on P₁-P₂ and w in a particular way: the specification of this dependence is called 'the scaling law';
(iii) 'thermal light sources' almost always obey the scaling law, and radiate spectrally-invariant light;
(iv) When the scaling law is violated as it is in the Lloyds mirror experiment I described, the spectrum is not invariant under propagation.

Although the effects observed in that 2-slit experiment are easy to understand, the statements I have just made seem rather more portentous.  If they lead to no stranger effects than those in my second year U/G experience, we'd be inclined to agree that maybe coherence theory is a stuffy and pretentious way of describing things that are physically rather obvious.

But wait ...