- #1

- 2

- 0

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter plato_2
- Start date

- #1

- 2

- 0

- #2

- 1,102

- 6

- #3

mgb_phys

Science Advisor

Homework Helper

- 7,774

- 13

Except in very unusual circumstances or with very high power beams there is no effect.

- #4

berkeman

Mentor

- 59,309

- 9,456

Except in very unusual circumstances or with very high power beams there is no effect.

We also talked more about this very subject in this PF thread:

https://www.physicsforums.com/showthread.php?t=222767

- #5

- 7,579

- 2,229

You have described a Mach-Zender interferometer.

- #6

- 2

- 0

So, I can use a Fresnel Prism as a beam splitter?

- #7

- 7,579

- 2,229

- #8

- 104

- 0

- #9

- 7,579

- 2,229

I don't understand what you are asking.

- #10

- 104

- 0

- #11

- 35

- 0

Or am I way off on this....

- #12

- 7,579

- 2,229

xArherx, photons do not travel like little bullets- that's a analogy that may be initially helpful but ultimately is internally inconsistent and leads to confusion. A single photon is of infinite extent, for example.

- #13

- 35

- 0

- #14

- 7,579

- 2,229

Real wavefronts have sptially and temporally limited extent. One way to handle this is to let many electromagnetic excitations form a 'wave packet', which consists of many photons, all with slightly different frequencies (energies), thus allowing spatial and temporal localization (think Fourier transforms).

In this way, detection of a photon is the collapse of the wavefunction.

- #15

- 35

- 0

- #16

- 104

- 0

- #17

- 29

- 0

xArherx, photons do not travel like little bullets- that's a analogy that may be initially helpful but ultimately is internally inconsistent and leads to confusion. A single photon is of infinite extent, for example.

Is air nonlinear enough for combining frequency? With some very high intensity, thats surely possible (even vacuum has some nonlinear properities)... but with some normal lasers - i though you need some nonlinear material for effective (visible) mixing.

- #18

- 35

- 0

I guess what I was thinking was that if you had two highways that intersected each other. Then cars drove through without looking, not every car will collide with another. Some of the cars will make it through the intersection unscathed. Now have many cars and the chance of a collision increases. Now make the cars really small and the chance of collisions decreases.

I'm curious about the mass part. Photons have no rest mass. However do they have a relativistic mass? Thinking of it classically, light is affected by gravity (black holes) but only object having mass is supposed to be affected by gravity.

So do they have a relativistic mass or do they remain massless even at relativistic speeds (meaning gravity affects them by some other reason)?

- #19

Cthugha

Science Advisor

- 1,965

- 333

Absolutely, cars will collide. Photons can mix/constructive interference/destructive interference.

You have to be careful here. It is not the case, that any arbitrary photons show constructive or destructive interference, if you superpose them. They usually have to come from the same coherence volume. If any light produces interference, you would see interferences, if you just pointed two flashlights at any screen, which is not the case.

- #20

- 35

- 0

Although shining two flashlights onto the same spot does cause the spot to be brighter than either individual flashlight, would that mean you are simply getting a constructive interference without the destructive? Or is it simply a larger quantity of photons?

I've always been fascinated and puzzled by light.

- #21

- 7,579

- 2,229

Is air nonlinear enough for combining frequency? With some very high intensity, thats surely possible (even vacuum has some nonlinear properities)... but with some normal lasers - i though you need some nonlinear material for effective (visible) mixing.

The mixing occurs at the detector- heterodyne detection (the beat frequency).

- #22

Cthugha

Science Advisor

- 1,965

- 333

Although shining two flashlights onto the same spot does cause the spot to be brighter than either individual flashlight, would that mean you are simply getting a constructive interference without the destructive? Or is it simply a larger quantity of photons?

In this case, there is no interference. As you said, there is just an increase in the number of photons due to having two flashlights instead of one.

What is meant by "same coherence volume"? Would a laser be an example of such, where the beam passes through two slots creating an interference pattern on a back screen? I've always wondered about that experiment and the shining of two flashlights is why. As you said, if you shine flashlights at the same spot there should be an interference pattern but there isn't one. How is it that the light coming from the same source shining through two slots produces the pattern while the light coming from two difference sources (kinda representing the light as it comes from each slot) doesn't? Also, I always thought that the intensity (brightness) of a light is proportional to the quantity of photons emanating from the source (or reflection). The interference experiment says that the increase and decrease in intensity is due to constructive and destructive interference. Is it one, or the other, or both?

To explain interference, you have to take the underlying fields into account. From a classical point of view, intensity (corresponding to the number of photons) is roughly speaking the square of the underlying em-field. The field is characterized by amplitude and phase. In order for interference to occur, we need the superposition of several fields with a fixed phase relationship, so the fields can add up (when in phase) or cancel (when in antiphase). Taking any arbitrary fields, the phase relationship will be random, so no interference will occur. You just add the squares of the single fields.

If the origin of the two fields is the same source however - like from two atoms inside the same laser - there might be a fixed phase relationship, which allows the fields to add up or cancel and thereby produces interference. So one can define an area of the light source within which there is a fixed phase relationship (you can imagine, that the fields from atoms next to each other have a fixed phase relationship where atoms, which are e.g. 2 meters apart, do not) and some delay, during which interference will occur (if you split a coherent beam, delay one of the arms and recombine them, you can see interferences for short delay times, where for long delay times the phase relationship is random again). So the distance photons can travel during the maximum delay, which still shows interference, times the area mentioned before gives you some kind of volume, which is a first estimation of what a coherence volume is. Generally speaking, the more monochromatic a source is, the larger the coherence volume will be.

From a more mathematical point of view, there is no interference, if the sum of the squares of the single fields equals the square of the sum of the single fields and interferences occur, if the square of the sum of the single fields shows additional terms.

- #23

- 35

- 0

- #24

- 7,579

- 2,229

Combine them, and you get a coherence volume, a cylinder of area = spatial coherence and length = temporal coherence.

A fluorescent source has low sptail and temporal coherence. The sun has poor temporal and moderate spatial coherence, while distant stars have quite high spatial coherence. A point source is perfectly spatially coherent, and a monochromatic source is perfectly temporally coherent.

- #25

- 35

- 0

Share: