Why does light not propagate backward in dense media?

It is easy to see how light propagating in a dense medium destructively interferes laterally and constructively interferes in the forward direction. This is why light will travel forward in such a medium but wont scatter laterally. This phenomenon is usually shown using a diagram like this one

or this one

However in both cases light appears to be constructively interfering in the backward direction as well as the forward direction. This would also make sense taking the conservation of energy into consideration. The incoming radiation incident on a particular layer of atoms constructively interferes going forward causing an increase in forward intensity. This increase in forward intensity must be offset by a decrease in lateral intensity on the one side (say the right side) . The backscattering interferes constructively leading to an increase in backward intensity. This increase in intensity is offset by a decrease in intensity on the other side (say the left side this time).

This must however be wrong though since light travelling through thick glass does not appreciably back-scatter compared to the forward propagation. So how does destructive interference take place in the backward direction when the diagrams I have shown seem to indicate that it should be constructively interfering in that direction?

Any help on this would be most appreciated!

• Commented Jul 4, 2022 at 3:20
• Also see if you want to keep huygens principle as a physical feelong huygens wave propagation corrected Commented Jul 4, 2022 at 3:26
• But basically, the equation for the backward wave is the same, so the huygens principle do not exclude it, as it is a principle for this equation. Only the disturbance you would solve goes back in time. The wave backward is actually the same wave but in the past (i.e. going back in time). Note that the initial condition includ the heigh of the disturbance but also its 2 first derivatives. Commented Jul 4, 2022 at 3:31