Whispering gallery to the walls, an effect that was discovered in the whispering gallery of St Paul's Cathedral in London. The extent to which the sound travels at St Paul's can also be judged by clapping in the gallery, which produces four echoes. Other historical examples are the Gol Gumbaz mausoleum in Bijapur and the Echo Wall of the Temple of Heaven in Beijing. A hemispherical enclosure will also guide whispering gallery waves. The waves carry the words so that others will be able to hear them from the opposite side of the gallery. The gallery may also be in the form
Whispering-gallery wave acoustic diffraction (lateral wave spreading) are then completely compensated. Whispering-gallery waves exist for light waves. They have been produced in microscopic glass spheres or toruses, for example, with applications in lasing, optomechanical cooling, frequency comb generation and sensing. The light waves are almost perfectly guided round by optical total internal reflection, leading to Q factors in excess of 10 being achieved. This is far greater than the best values, about 10, that can be similarly obtained in acoustics. Optical modes in a whispering gallery resonator are inherently lossy due to a mechanism similar to quantum tunneling. As a result, light