Fizyka dla każdego

Congratulations to 김예진   &   고다현 

You are right!

고다현, You are right: - the falling pendulum moves around the circle, with constant velocity, see film below

김예진, You are right: the energy is conserved in the destructive interference, see explanation below

This MIT video is instructive and a real experiment that shows that in destructive interference set up with interferometers there is a return beam, back to the source, as far as classical electromagnetic waves are considered. So the energy is balanced by going back to the source.

What is happening at the photon level? The quantum mechanical solution with the complicated boundary values of the interferometer allows the elastic scattering of photons also back to the source. You can see in the video that there always exists a beam going back to the source, that beam is carried by individual photons scattering elastically backwards through the system of the optics of the interferometer. In total destructive interference all the energy is reflected back (minus some due to absorption and scattering in the matter of the optical system).

In order to get an interference pattern, the photons have to react with a beam splitter, or some other matter, as in the laser experiment. The reason that matter is needed for light interference phenomena is due to the very small electromagnetic coupling constant. Photon-photon interactions have a very small probability of occurrence  with respect to photon-electron interactions. To all intents two laser beams crossing will go through each other without any measurable interaction (interferences included) if no matter is present.

For mechanical waves: in the case of two sound waves interfering destructively, the temperature of the medium will go up and energy is conserved because it turns into incoherent kinetic energy of the molecules of the medium.

The two problems were solved by: - mgr Krzyś Służewski and dr Kamil Fedus

See other photos from the lecture