Ask A Scientist Physics Archive

name         Ilian T.
status       educator
age          30s

Question -   What will happen when one splits a laser beam in two
parts of equal(not NECESSARILY) amplitude and then collect them again in
the a same direction with a lambda/2 path shift? The beams should
extinguish each other - negative interference? Now:
What about Energy conservation law?
If light returns back in the laser what is the path where it goes and why?
If it has a positive interference direction where is it and is the  balance
for the energy OK.
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Good question!  It turns out that it is always impossible to make the
two light beams overlap completely in a region where they are lambda/2
out of phase.  There are other regions where they are in phase and
this, of course, always works in such a way that energy is conserved.

Consider, for example, two slit diffraction patters.  On the screen
There are dark and light bands corresponding to negative and positive
interference.

Light reflected back into a laser will interfere with the light in the
laser producing positive and negative interference patters.  Once
again, you can rest assured that energy is conserved.

Best, Dick Plano
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