Breaking EM Destructive Interference

[MTtylerb]

Hi, I have a question about destructive interference. Suppose I have two very closely spaced EM sources, e.g two RF emitters. At a point far away from the sources the arbitrarily orientated magnetic field due to source one is: B1(t)=sin(ωt) and source two: B2(t)=sin(ωt+pi) so the total field is null due to destructive interference B(t)=B1(t)+B2(t)→0 (the same is true for the electric fields). Is there anyway to extract energy from the nonzero energy wound up in the individual fields? Or does destructive interference of the two fields make this impossible?

 

[Dale]

Hi MTtylerb, welcome to PF!

You can extract energy from the field at other locations, just not from the locations where there is complete destructive interference.

 

[MTtylerb]

What if the interference occurs everywhere in space?

 

[Drakkith]

What if the interference occurs everywhere in space?

This is not possible, there must be areas with constructive interference.

 

[MTtylerb]

How about two infinitely long solenoid coils wound in between another but with opposing currents? The net fields throughout space would be null as it is composed of the non-zero individual fields.

 

[Dale]

If you wind them in between one another then you will still have regions of constructive superposition. It is simply impossible to have destructive interference everywhere in space.

 

[chrisbaird]

The term "destructive interference" can be misleading when you are talking about energy. A better name might be "energy redirection". The fields of the waves add together mathematically to destroy each other at certain points, but there is no actual physical destruction happening. The energy is redirected. (Waves can destroy objects, such as a laser cutter, but this is through high-energy absorption, not through wave interference.) This energy redirection can be quite useful. For example, frequency-selective coatings on glass can be made using interference. Certain frequencies are reflected instead of transmitted.