While the usual Doppler effect means that the frequency increases if the observer approaches the source – and decreases as they move away from each other – theorists have speculated, since 1943, about the possibility that these rules may be interchanged. That would create an inverse Doppler effect.
An experiment that claimed to have detected this effect was conducted by Nigel Seddon and Trevor Bearpark in Bristol, United Kingdom in 2003.
Researchers from Swinburne University of Technology and the University of Shanghai for Science and Technology showed that this effect can be observed in optical frequencies as well. This was made possible by growing a photonic crystal and projecting a laser beam into the crystal. This will make the crystal act like a super prism and the inverse Doppler effect can be observed.
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