Phenomena of reflection of light

[thamwenyin]

I wonder why is there a gap at the starting of the reflected light when i conduct this experiment? Can anybody explain this to me? Thanks!

 

[Andy Resnick]

Your attached thumbnail makes no sense; are you somehow referring to the Goos-Hanschen effect?

http://en.wikipedia.org/wiki/Goos–Hänchen_effect

AFAIK, this is a significant effect when transmission in long optical fibers is involved, but not much else.

 

[Redbelly98]

I wonder why is there a gap at the starting of the reflected light when i conduct this experiment? Can anybody explain this to me? Thanks!

Welcome to Physics Forums.

Can you describe your experiment a little more? Specifically, exactly how are you "observing" the light ray or beam?

 

[Redbelly98]

If I had to bet, I would guess that you are using a card or sheet of paper to see where the light beam is. When you hold the sheet near the reflecting surface, it is actually blocking the incident beam, making it appear that there is no reflected beam right near the surface. But in reality there is no reflected beam because the incident beam is blocked before it reaches the surface.

Just a hunch.

 

[thamwenyin]

If I had to bet, I would guess that you are using a card or sheet of paper to see where the light beam is. When you hold the sheet near the reflecting surface, it is actually blocking the incident beam, making it appear that there is no reflected beam right near the surface. But in reality there is no reflected beam because the incident beam is blocked before it reaches the surface.

Just a hunch.

The experiment that I conducted is similar to this>>http://www.youtube.com/watch?v=LMmnuUOZ6ho&feature=related

From the video,it is obvious that there's a gap between the plane mirror and starting of reflected ray. Why does this happen? Is it caused by the glass and the coating of reflective metal in the mirror? But how?

 

[Dadface]

Why does this happen? Is it caused by the glass and the coating of reflective metal in the mirror? But how?

It is caused by the glass and the reflective coating.Most of the light incident on the mirror enters the glass ,gets refracted towards the normal,gets reflected bythe reflective coating and emerges,after being refracted away from the normal at the surface.With the type of mirror and/or experimental set up you used you were unable to see the rays in the glass itself so this shows up as an apparent gap.Not all of the light is reflected by the coating some of it being reflected by the front surface of the mirror.In fact there are multiple reflections.If you look very carefully you may be able to see the weak reflected rays due to the first reflection from the front surface of the mirror.

 

[thamwenyin]

It means that the gap is caused by few times of reflection and refraction? I am still blur about how reflection and refraction of a light in the mirror stacks and come out with a gap.

 

[solerFF]

is it got something to do with the destructive of wave?

 

[Andy Resnick]

The experiment that I conducted is similar to this>>http://www.youtube.com/watch?v=LMmnuUOZ6ho&feature=related

From the video,it is obvious that there's a gap between the plane mirror and starting of reflected ray. Why does this happen? Is it caused by the glass and the coating of reflective metal in the mirror? But how?

ok- I see what you are referring to.

The details of the setup are not clear in the video (I wasn't listening to any audio track)- for example, why does the light appear to pass through to the person's thumb?

If the mirror is off the table surface, the gap could appear simply from geometry- the rays of light are diverging (spreading) normal to the table, and so if the mirror is off the table surface, the reflected light won't hit the table for a short distance.

That's just a guess, tho.

 

[Redbelly98]

It is caused by the glass and the reflective coating.Most of the light incident on the mirror enters the glass ,gets refracted towards the normal,gets reflected bythe reflective coating and emerges,after being refracted away from the normal at the surface.

I don't think so. If that were true, there should be a gap in both the incident and reflected rays. But if you pause the video when they show the more extreme angles, it pretty clearly looks as posted in the OP. It seems most evident when the incident and reflected beams are at 90 degrees to each other: a gap in the reflected ray, whereas the incident ray extends completely to the point where the two lines intersect.

I'll repeat my earlier question to the OP: exactly how are you "observing" the light ray or beam?

 

[thamwenyin]

I'll repeat my earlier question to the OP: exactly how are you "observing" the light ray or beam?

I put the ray box and the mirror on top of a sheet of smooth white paper and observe the light ray from a plan view. I tried removing the paper and observe it on the table,the same thing happened.

 

[solerFF]

could a front coated mirror and a rear coated mirror brings any significant difference to the experiment?

 

[Dadface]

For a rear coated mirror I think that what I wrote in post 6 holds,the gap appearing to be between the incident and reflected rays and displaced laterally(depending on angle of incidence) as well as along the line of the normal.You can see this quite clearly by using a ray box a sheet of white paper and a non coated thin glass block as a mirror.
Redbelly,How do I pause the video?I need to look at it more carefully but it seems to be a front coated mirror.On second thoughts......

 

[Dadface]

I think I see it.Firstly there is obviously a fairly strong reflection but the incident rays seem to travel on a short distance before stopping.This is due to those rays which are passing over the top surface of the mirror and being scattered(in the same way that the rays are made visible by scattering on the white paper).In a lot of views those rays are not visible beyond the back surface of the mirror because the mirror itself has blocked/reflected those rays which are grazing the surface.Those "higher"rays do pass beyond the mirror,however,as evidenced by those views where the experimenters hands lie in the path of the incident rays.Take another look at the video and you will clearly see the rays passing over the experimenters fingers.I think the apparent lateral displacement is due to reasons outlined in post 6.

 

[Redbelly98]

Redbelly,How do I pause the video?

To the lower left of the video frame, there is a pause button (two vertical lines in a box) and a volume control button. Pause at about 21 or 22 seconds for a good view.

 

[Dadface]

Thanks Redbelly,pausing enabled me to see things more clearly but there was nothing that surprised me.I think we need a more detailed description of the experimental set up, this to include the type of mirror used,its height and the height of the slits used in the ray box.

 

[solerFF]

actually... recently I've conducted a similar experiment using varying heights of mirror. It still yields a similar void at the light ray with nothing blocking the light.

 

[sophiecentaur]

An interesting question which is seldom raised. I have thought about this and this is my explanation.
The answer is, I think, in the simple geometry of the setup. Any of those types of demonstrations are not, in fact, using 'rays'. They are using the light coming from a vertical slit source, which is a vertical band or ribbon of light. Each patch of light on the paper is produced by light from a higher and higher part of the ribbon (If it were a single ray, it would get no further than the first few bumps and hairs on the paper). The part of the vertical ribbon of light which hits a surface silvered reflector will hit the paper immediately in front of it and there would be no gap. With a normal rear-slvered mirror, however, there is a narrow part of this vertical ribbon which enters the glass and is not reflected at the glass surface. You have to look a bit further up the vertical ribbon until you get some light that will be reflected off the rear silvering. So a small slice of the incident ribbon gets lost* in between the glass surfaces. Because it is such a shallow angle, the effect is exaggerated, the start of what we interpret as the reflected ray is not there and a small gap is seen.

*There will be some total internal reflection off the bottom glass surface which will totally lose the light (sending it upwards) that would have formed the first bit of the reflected 'ray'.

 

[thamwenyin]

So a small slice of the incident ribbon gets lost* in between the glass surfaces. Because it is such a shallow angle, the effect is exaggerated, the start of what we interpret as the reflected ray is not there and a small gap is seen.

*There will be some total internal reflection off the bottom glass surface which will totally lose the light (sending it upwards) that would have formed the first bit of the reflected 'ray'.

When i conduct the experiment,by using different angles of incident varying from >0 to 90<,the gap still appears. Based on what you said,total internal reflection in the mirror cause the gap to manifest, but total internal reflection will happen only at certain angle, how the gap can be seen all the time with all the angles i tried?

 

[sophiecentaur]

But, whatever the angle, the 'ribbon' is still not complete, is it? I only mentioned the TIR bit as an afterthought.
I can't imagine the phenomenon is due to any diffraction mechanism - the gap is far too big - so it must be due to geometry.

I shall sit down (you too?) and try some multiple ray diagrams and see what happens in detail.

I could ask whether the missing bit is the same width for all angles in your experiment. If you were to use a thin (laser) beam, we'd expect the same missing portion.

 

[Dadface]

There is a double effect with a rear coated mirror and with the apparatus I suspect was used and I don't think I explained myself clearly enough(posts 6 and 14).Let me try again:

1.The rays get refracted on entering the mirror,then reflected at the silvered surface and get refracted again on emerging from the mirror.Only the rays scattered from the paper surface are clearly visible,not those within the mirror,but the incident and emergent rays only.The result is that for non normal incidence there appears to be a gap,the incident and emergent rays being laterally displaced from each other.
2.Those parts of the rays grazing the top surface of the mirror get scattered by the mirror itself giving the impression that the incident rays are not refracted but travel on linearly by a short distance which is dependant on the angle of incidence and thickness of the mirror.
If you use a laser beam that does not graze the mirror surface or slits in the ray box which are shorter than the mirror then this should eliminate effect number two.A mirror with a white scattering coating along one edge and which should be suitably placed will show the refracted rays(ideally using a ray box because it would be awkward to get the laser lined up properly) and eliminate effect number one.Alternatively,a plastic block with one side coated can be used in place of the mirror.Such blocks are readily available.

Sophiecentaur some of our ideas seem to be similar but remember that total internal reflection can occur only on going from a more dense to a less dense medium.With this experiment only those rays which are reflected towards the mirrors edges and with large enough angles of incidence can be totally reflected.

 

[Redbelly98]

thamwenyin, here are two more possibilities:

Is the mirror reflective all the way to the edge of the mirror, or is there a small part of the mirror, at it's edge, that does not reflect?

Is the mirror's edge touching the paper, or is it positioned slightly off of the paper?

My thinking is, if either the mirror is not reflective close to it's edge, or if there were a small gap between it and the paper, then you would have a gap between the mirror and where the reflected ray first hits the paper.

 

[sophiecentaur]

There is a double effect with a rear coated mirror and with the apparatus I suspect was used and I don't think I explained myself clearly enough(posts 6 and 14).Let me try again:

1.The rays get refracted on entering the mirror,then reflected at the silvered surface and get refracted again on emerging from the mirror.Only the rays scattered from the paper surface are clearly visible,not those within the mirror,but the incident and emergent rays only.The result is that for non normal incidence there appears to be a gap,the incident and emergent rays being laterally displaced from each other.
2.Those parts of the rays grazing the top surface of the mirror get scattered by the mirror itself giving the impression that the incident rays are not refracted but travel on linearly by a short distance which is dependant on the angle of incidence and thickness of the mirror.
If you use a laser beam that does not graze the mirror surface or slits in the ray box which are shorter than the mirror then this should eliminate effect number two.A mirror with a white scattering coating along one edge and which should be suitably placed will show the refracted rays(ideally using a ray box because it would be awkward to get the laser lined up properly) and eliminate effect number one.Alternatively,a plastic block with one side coated can be used in place of the mirror.Such blocks are readily available.

Sophiecentaur some of our ideas seem to be similar but remember that total internal reflection can occur only on going from a more dense to a less dense medium.With this experiment only those rays which are reflected towards the mirrors edges and with large enough angles of incidence can be totally reflected.

1. I think the only effect of the refraction is to bring the effective position of the silvering a bit further forward.
2. Using a laser beam is not likely to show the whole 'ray picture' at the same time as different bots of the 'ribbon' show different portions of the ray picture.
3. Something also struck me, after having drawn a few diagrams. Because it's a mirror, there is inversion and it is the lower bits of the ribbon which hit the paper on the way out (edit: IN!) and the higher bits which trace out the pattern in the reflected direction.
4 Thinking in 3D, dadface, is a good idea and it certainly shows how there will have to be a gap between the end of the incidence line and the reflection line because what goes on within the glass won't be (as) visible - I still think that TIR is relevant to reducing the visibility of the paths inside the glass - it works up to 40 degrees or so. (Actually more than the critical angle, due to the bending at the front surface)

 

[Redbelly98]

There is a similar effect in this demo of a lens by the same person. I suggest pausing the video at about 2:11:



In this video, we don't have to wonder about front surface vs. back surface mirrors. The ray paths are pretty unambiguous.

 

[Redbelly98]

FWIW, I have taken video snapshots. I think it's pretty clear that the gap is not caused by having a back-surface reflector:

And from the convex lens demo by the same person (people?):

 

[Gear300]

I looked through some additional pictures and noticed the same thing occur in a good number of them (coming from both a reflection and a refraction). Could it be from a phase change by reflection (though, I'm not sure how well this would apply to the refractive case)?

 

[Redbelly98]

Why would a phase change make the beam invisible? And why would that effect magically go away after a few mm?

Whatever is being used to view the beam -- apparently a sheet of paper, from what others have posted -- does not have any light from the reflected beam hitting it, within a few mm or so after reflecting off of the mirror. There is light coming off of the mirror, it is just not hitting the paper sheet right away.

 

[Gear300]

Whatever is being used to view the beam -- apparently a sheet of paper, from what others have posted -- does not have any light from the reflected beam hitting it, within a few mm or so after reflecting off of the mirror. There is light coming off of the mirror, it is just not hitting the paper sheet right away.

Oh...that makes more sense.

 

[Dadface]

With the first snapshot in post 25 assume that the line joining the the beginnings of the reflected rays is the front surface of the mirror and further assume that the relevant parts of the incident rays stop at this line.In the first instant consider the line only and ignore those parts of the incident rays that travel on a short distance beyond the line.It may help to hold a piece of paper along the line in order to block those parts of the incident rays that travel on.Now what is seen is exactly what would be seen by a back surface reflector.Each incident ray refracts on entering the mirror and refracts again on emerging from the mirror resulting in the lateral displacements at the surface revealed in the snapshot.The refracted rays are not visible with this mirror because its base which is on the paper is not coated with a white scattering surface. Base coated blocks ,lenses and prisms are readily available but not mirrors(well I haven't seen them.I guess there is not a demand for them).I think the main thing that confuses this snapshot is the fact that the incidence rays seem to move on linearly and by a further short distance but this is due to those "higher" parts of the rays which are grazing and getting scattered by the top of the mirror.Further evidence that rays are passing over the top of the mirror is provided by the light scattered by the experimenters hands.Take a look.

Snapshot two is using a base coated cylindrical lens which shows the refracted rays inside the lens and because of refraction these rays to be closer to the surface than they actually are(real and apparent depth phenomenom).This results in a real gap but it is vertical(along the lines of the normals to the top and bottom surfaces)rather than horizontal.The size of the gap seen depends on the viewing angle and for a perfect plan view no gaps will be visible the incident ,refracted and emergent rays all terminating at the edges of the lens.The video from which the snapshot is taken shows clearly that there is no visible gap for front surface reflection(see,for example 28 to 31 seconds)

 

[thamwenyin]

Thanks for all your replies! I learned a lot!