What Bends Light

[BillyJoe]

How many different ways are there to bend light?

By bending, I mean either bend sharply at an angle or bend gradually (curve). I have a reason for asking this but I want to be sure I'm not missing anything.

Thanks for any contributions.

BillyJoe

 

[LuxFerum]

Diferents rates of refractive index , and distortions on time and space, although I´m not sure if the second counts as bending light.

 

[epepke]

How many different ways are there to bend light?

By bending, I mean either bend sharply at an angle or bend gradually (curve). I have a reason for asking this but I want to be sure I'm not missing anything.

Thanks for any contributions.

BillyJoe

A ridiculously large number of ways, but two main ones.

Gravity bends light relative to most observers, but it's still travelling along a geodesic.

Anything that can stop or reduce the frequency of the amplitude of light can bend it. Glass, water, black paint, acousto-optical modulation, etc.

 

[Mojo]

And then there's Uri Geller, of course...

 

[Badly Shaved Monkey]

A ridiculously large number of ways, but two main ones.

Gravity bends light relative to most observers, but it's still travelling along a geodesic.

Anything that can stop or reduce the frequency of the amplitude of light can bend it. Glass, water, black paint, acousto-optical modulation, etc.

Why does refraction bend light not just slow its progress in the same direction?

On a tangentially related subject why does light spread from a diffraction slit? If a beam is shot at the slit one photon at a time it gives a smeared out distribution on a screen the other side. How does it 'know' its been through a slit and so has to spread out instead of just carrying straight on.


I have no problem with interpreting the two-slit experiment, but the single slit that we were supposed to take for granted always left me a bit nonplussed.

 

[Terry]

I have no problem with interpreting the two-slit experiment, but the single
slit that we were supposed to take for granted always left me a bit
nonplussed.

Disclaimer: not a physicist. One way of looking at it is that light goes in
straight lines because the amplitudes (quantum equivalent of probability,
roughly speaking) for it to take other paths cancel out. This is Feynman's
"sum over paths" approach. So if you put in a slit, many of the possible paths
are now blocked, so their amplitudes no longer contribute to the path after
the slit. Hence the amplitude for the photon to be off the straight line
increases.

--Terry.

 

[CurtC]

Why does refraction bend light not just slow its progress in the same direction?

On a tangentially related subject why does light spread from a diffraction slit?

Obviously, you think of light as a ray, or a particle. This photon, to your mind, is just a little bullet that should want to go in a straight line.

The dual nature of matter is not intuitive, but if you instead think of light as a wave, these effects make sense. Imagine you have a sea wall with a small gap, and on one side you hit it with a parallel wave. When the wave gets to the gap, it will propagate inside the sea wall radially from that point. That's intuitive, right?

About refraction, you could also see the same effect with liquid waves, but we don't have as much experience with different media other than water. But here's a way to imagine it - think of a sinusoidal (in time) wave, where the waves are parallel with each other and perpindicular to their direction of travel. Now draw a line at the peak of each wave, so it looks like parallel lines, perpindicular to the direction of travel. The lines are spaced apart a distance that depends on the frequency of the wave, and the speed of the medium. Now imagine that hitting another medium that propagates the wave at a slower speed, and hitting it at an angle. The frequency is still the same (cycles per second), so the distance between the peaks will be smaller. The places where your drawn lines intersect the boundary will be peaks of the wave on both sides, so the only way the waves can be closer together would be for them to "bend" in a new direction.

Next subject: holograms!

 

[wollery]

Why does refraction bend light not just slow its progress in the same direction?

It will, but only if the incident beam is normal to the surface of the refracting material. Otherwise the wavefront is turned by being slowed at one edge first.

On a tangentially related subject why does light spread from a diffraction slit? If a beam is shot at the slit one photon at a time it gives a smeared out distribution on a screen the other side. How does it 'know' its been through a slit and so has to spread out instead of just carrying straight on.


I have no problem with interpreting the two-slit experiment, but the single slit that we were supposed to take for granted always left me a bit nonplussed.

I don't get why the two slit experiment makes sense to you, but the single slit doesn't. The two slit result only works because of interference between the diffractions from each single slit.

 

[epepke]

Why does refraction bend light not just slow its progress in the same direction?

For a complete answer, you'd need to look at Quantum Electrodynamics. For an approximate answer, you can use wave theory or an analogy. When light hits a surface at an angle, one side is slowed down first, and so it turns.

Of course, this doesn't really happen in the same sense that if one tread of a tank stops, it turns. But the amplitudes on one side slow down in frequency. The path of light is given by the sum of the amplitudes.

On a tangentially related subject why does light spread from a diffraction slit? If a beam is shot at the slit one photon at a time it gives a smeared out distribution on a screen the other side. How does it 'know' its been through a slit and so has to spread out instead of just carrying straight on.

Again, a complete answer relies on QED. To do a calculation, you have to consider all possible paths the light can go. Straight, wiggly, bent, looping around, whatever. Each path has an amplitude which, ignoring polarization, can be visualized as a vector spinning around at a certain rate given by the "wavelength" of the light and the distance traveled. Light goes where nearby amplitudes add up as a vector quantity.

With a slit, the solid material makes certain paths impossible. The paths that go through the opening are still possible. Near the edge of the slit, some of the amplitudes of possible paths are unimpeded, and some are impeded. They sum up differently, which may produce another angle of maximum probability.

You should back off a bit and ask why light travels in a straight line in the first place. Or why anything does. Maybe you take this for granted, but you shouldn't.

The answer is in QED. Near a straight line, there are a lot of nearby paths that are very close, and so the amplitudes of those paths add up. The maximal probability is a straight line.


I have no problem with interpreting the two-slit experiment, but the single slit that we were supposed to take for granted always left me a bit nonplussed.[/QUOTE]

 

[AmateurScientist]

Bender

 

[kalen]

Compton scattering is another way when you consider individual photons.

Also, photons, if their energy is just so, can excite an atom (or molecule). When the atom reurns to the ground state, the photon that is emitted will usually go off any old direction. The same situation applies to nuclear excitations.

 

[ReFLeX]

I have a reason for asking this but I want to be sure I'm not missing anything.

So! Why do you ask?

 

[Ririon]

Why does refraction bend light not just slow its progress in the same direction?

Why do the waves always roll nicely inward to the beach instead of in some cases moving along the coastline depending on the wind direction?

 

[69dodge]

Again, a complete answer relies on QED. To do a calculation, you have to consider all possible paths the light can go. Straight, wiggly, bent, looping around, whatever.

I've never quite understood this sort of phrasing, although it's very common. The problem is, we can't really interpret "all possible paths the light can go" too literally. In QED, photons don't follow any path; they are emitted and some time later they are absorbed. And that's it. "All possible paths the light can go" really means something like, "all possible paths the light could go, were it a classical particle, which we know it isn't."

Do you agree?

 

[BillyJoe]

So! Why do you ask?

This is a drawing made by a witness after he described to an investigator what happened to the headlights of his car as he passed a UFO on the side of the road.

The question is:
How could the light be bent in the manner described?


Note:
There are two beams of light, one from each headlight. Each beam is bent twice, once as it leaves the headlight and a second time as it reaches the "imaginary line"
The "imaginary line" is a line drawn between the UFO (on the right side of the road) and a tree (on the left side of the road). The "imaginary line" crosses the road at right angles. Notice that the angle between the "imaginary line" and the beam bending to the right decreases as the car approaches the "imaginary line", but the angle between the imaginary line and the beam bending back to the left remains constant.


Any ideas?

(Remember, there HAS to be a way, because it DID happen. So you are not allowed to dismiss the witness as a crank, hoaxer, delusionary etc etc. The witness knows what he saw. )


BillyJoe

 

[sphenisc]

This is a drawing made by a witness after he described to an investigator what happened to the headlights of his car as he passed a UFO on the side of the road.

The question is:
How could the light be bent in the manner described?


Note:
There are two beams of light, one from each headlight. Each beam is bent twice, once as it leaves the headlight and a second time as it reaches the "imaginary line"
The "imaginary line" is a line drawn between the UFO (on the right side of the road) and a tree (on the left side of the road). The "imaginary line" crosses the road at right angles. Notice that the angle between the "imaginary line" and the beam bending to the right decreases as the car approaches the "imaginary line", but the angle between the imaginary line and the beam bending back to the left remains constant.


Any ideas?

(Remember, there HAS to be a way, because it DID happen. So you are not allowed to dismiss the witness as a crank, hoaxer, delusionary etc etc. The witness knows what he saw. )


BillyJoe

Can you give us the labels on each of the diagrams ? Any idea of the speed of the car? Is the UFO stationary?

Thanks

 

[69dodge]

On a tangentially related subject why does light spread from a diffraction slit? If a beam is shot at the slit one photon at a time it gives a smeared out distribution on a screen the other side. How does it 'know' its been through a slit and so has to spread out instead of just carrying straight on.

I have no problem with interpreting the two-slit experiment, but the single slit that we were supposed to take for granted always left me a bit nonplussed.

The same way that a photon that goes through one slit can know that a second slit is present, it can know that a second slit is absent.

Imagine that the "natural" behavior of a photon is to spread out from a slit in a barrier, rather than to travel in a straight line where there's no barrier. And imagine that "no barrier" is really infinitely many successive barriers with infinitely many slits in each. Then, the reason light appears to travel in a straight line where there's "no barrier" is really that the light spreading from all the slits of each barrier interferes.

This is the same as what epepke said, just in different words.

 

[Badly Shaved Monkey]

When the wave gets to the gap, it will propagate inside the sea wall radially from that point. That's intuitive, right?

Actually, no. I've seen it often enough to accept it as normal, but it isn't really intuitive. It begs all sorts of questions about how the wave (water or light) 'knows' that there is an edge just of to one side that will cause it to slow. If I extend the analogy to light waves it would imply some sort of friction between the lateral edge of the light wavefront and the edge of the slit and that this effect extends a significant distane off the surface of the material that constitutes the slit edge.

 

[69dodge]

BillyJoe, how does the witness know what path the light took? Was it foggy or something?

 

[force_redo]

(Remember, there HAS to be a way, because it DID happen. So you are not allowed to dismiss the witness as a crank, hoaxer, delusionary etc etc. The witness knows what he saw. )

Shame. That would have made it easier. ;-)

However, there might be "obvious" explainations in optics for this (hot street with hot air above. The "street seems to be wet in sunlight"-effect)

I can also think of an optical illusion that might occur if there's a bump (a rather large one) on the street:
Since the driver sits off center of at least one of the headlights and the light is cast over a bump this beam might look "bent" viewed from an angle.
(Similar to the "why do the shadows in the moon landing photograps seem to point in different directions"-problem)

Haven't tried, wether this really occurs and maybe it's a bit far fetched, but hey...

FR