# What'd happen if two light source collide w/ each other?

## Main Question or Discussion Point

Imagine this, there are 2 light source (laser perhaps) pointing each other with a distance between them. They have the same frequency, wavelength and amplitude. But, laser 1 start with positive amplitude while the laser 2 start with negative amplitude (start off from negative), what would happen when they collide with each other at the point between them?

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UltrafastPED
Gold Member
Your asking what happens when monochromatic light beams pass through each other in opposite directions, with opposite phases?

The situation is very unrealistic ... but here goes:

The beams will interfere destructively ... but for every location where the interference is destructive, there will be another where it is constructive. The energy just moves around.

You can easily illustrate this with beams that cross at a slight angle, or are not quite the same frequencies.

Yeah i know its very unrealistic...

I only can imagine if there are only constructive interference. Im sorry but i cant see where the destructive coming from.... My bad if i dont make myself clear, but my point is, what would happen when these light sources collide with each other at a point exactly between them (i imagine these light as a single sinusoidal wave).

And what would happen if they have the same phase? Would they cancel each other by destructive interference?

Thanks!

sophiecentaur
Gold Member
Imagine this, there are 2 light source (laser perhaps) pointing each other with a distance between them. They have the same frequency, wavelength and amplitude. But, laser 1 start with positive amplitude while the laser 2 start with negative amplitude (start off from negative), what would happen when they collide with each other at the point between them?
You are describing what happens in a standing wave, where a wave is continually reflected at two surfaces and the energy in the resonant space builds up, forming nodes and antinodes. There is a zero net flow of energy (as with your two 'manufactured' beams) but, whereas in a real standing wave, the energy goes nowhere outside the experiment, these two beams will need to be absorbed at either end of their paths. Unfortunately, this situation is not a realistic one to produce because, if the two beams are perfectly aligned - to follow your requirement - then they will each need to be absorbed in the other laser (you cannot do it with any arrangement of mirrors) and, as they need to have exactly the same frequency, each beam will contribute to the laser process in each of the lasers. What you would have would be a sort of double laser with two optical cavities. Where would the light light energy actually go if is perfectly absorbed at each end?

Claude Bile
Essentially, if a photonic mode is destructively interfered with then no photons will be emitted into such a mode. So basically, in the OP's ideal example the lasers will not emit photons at the wavelength where the interference exists. Instead the energy will be distributed to other wavelengths within the emission spectrum of the laser.

There are structures called photonic crystals that work on a principle like this. Many examples in literature exist on how emission properties of lasers and other emitters such as quantum dots have been modified using photonic crystals.

Claude.

sophiecentaur