A team of physicists working at the University of Glasgow has found a way to slow the speed of light that does not involve running it through a medium such as glass or water. Instead, as they explain in their paper published in the journal Science, they caused a change in the speed by first running it through a mask, which changed its shape.
Everyone knows that the speed of light in a vacuum is constant, but now it appears that there is a way to indirectly alter its speed—by running it through a special mask. Doing so apparently causes a change to the shape of the photon, making it move through a vacuum slower than an unaltered photon.
The researchers built what they called a racetrack—setting up a dual course for firing photons and detecting when they struck a detector a meter away. The first group of photons was fired in the normal way, while the second group was fired through a filter to shape it into either a Gaussian or Bessel beam.
Read more at: Phys.org