I Weird Lamp Effect: Investigating the Mystery of Fringes

[Jigyasa]

Hi everyone

So something strange happened tonight. I've always hung my night lamp on a hook in the wall but last night I kept it on the floor. And i saw fringes form on the floor (see picture)

Could someone please explain why the fringes formed? My guess is that two different waves from two coherent sources must be interfering somewhere but I'm not sure which sources. Is it the wave from the lamp and another reflected from the wall directly behind it? I'm not sure if the wall reflects light so strongly

 

[.Scott]

My guess is that those are simply patterns in the clear plastic parts of the lamp.
Do you have other pictures of the lamp - that show us some details.

 

[Merlin3189]

I wonder what sort of lamp it is? Interference seems unlikely.
I have a compact fluorescent with 6 vertical tubes arranged around in a cylinder and I get something like that (not nearly so pronounced) as each tube casts a shadow from the other tubes.

 

[Jigyasa]

My guess is that those are simply patterns in the clear plastic parts of the lamp.
Do you have other pictures of the lamp - that show us some details.

Attaching a few other pictures

 

[Jigyasa]

I wonder what sort of lamp it is? Interference seems unlikely.
I have a compact fluorescent with 6 vertical tubes arranged around in a cylinder and I get something like that (not nearly so pronounced) as each tube casts a shadow from the other tubes.

Hi Merlin
Mine also seems to have a couple of thin wire-like bulbs enclosed in a bigger cylinder. Attaching pictures for your reference

 

[.Scott]

So that pattern is coming from the green grass part.
That part looks as it is based on a 16-sided polygon. Not circular.

 

[Jigyasa]

So that pattern is coming from the green grass part.
That part looks as it is based on a 16-sided polygon. Not circular.

But why will the glass produce fringes? How does the polygon restrict light coming out from one region and allow it to come out the other?

 

[.Scott]

The surfaces on the glass are curved - and are acting like lenses. Where the curve is tight, the focal length is short and almost matches the distance to the filament - so that tends to create a bright band. Where the glass is relatively flat, the focal length is very large and the light is not deflected, so the brightness tails off as the distance increases - as it normally would.

Basically, each of the 16 bends is focusing some of the light into a vertical stripe.

 

[Jigyasa]

The surfaces on the glass are curved - and are acting like lenses. Where the curve is tight, the focal length is short and almost matches the distance to the filament - so that tends to create a bright band. Where the glass is relatively flat, the focal length is very large and the light is not deflected, so the brightness tails off as the distance increases - as it normally would.

Basically, each of the 16 bends is focusing some of the light into a vertical stripe.

Makes sense. But there is a problem. The curvature is the same along a horizontal line. So each point along the equator (if I can call the horizontal line at the middle that) should produce the same type of fringe (bright, in the case, because the curve is tightest at the equator). But we see that there are alternate light and dark fringes along a horizontal line

 

[.Scott]

The curvature is the same along a horizontal line. So each point along the equator (if I can call the horizontal line at the middle that) should produce the same type of fringe (bright, in the case, because the curve is tightest at the equator). But we see that there are alternate light and dark fringes along a horizontal line

By "horizontal line", I assume you mean a line radiating from the center of the filament to a point outside of the glass. A photon attempting to follow such a path will be diverted twice: once upon entering the glass; once upon exiting the glass. If it hits both of those surfaces at the same angle, then it will continue in the original direction (although perhaps offset slightly) and there will be no noticeable effect. But when that line is striking near one of the 16 curves, the light will hit the inside surface at a slightly different angle than the outside surface - causing the path to be deflected by some angle.

Those 16 curves are along the equator. If you take an equatorial section of that green glass, you will not get a circle. You will get a smoothed version of a 16-sided polygon. It is those 16 bends, each stretching from the top of the glass to the bottom, that are causing the stripes.

 

[Merlin3189]

It reminds me a bit of lighthouse Fresnel lenses. Here is the one from Pigeon Point California.

It has 24 panels producing 24 radial beams. By rotating the frame holding the lenses once every 4 minutes, the beams rotate such that a distant observer sees the light flash once every ten seconds as a beam swings by him.

 

[Jigyasa]

What shape are the 16 curves? They are just a depression on the inside so I am assuming I can treat them as plano concave lenses?

 

[Zdravko]

Make an experiment. See if you get the same pattern without the green glass part. If you do not. There is your answer.

 

[Jigyasa]

Make an experiment. See if you get the same pattern without the green glass part. If you do not. There is your answer.

I don't. I figured that. But I don't understand exactly how the pattern is formed by the green glass

 

[Jigyasa]

By "horizontal line", I assume you mean a line radiating from the center of the filament to a point outside of the glass. A photon attempting to follow such a path will be diverted twice: once upon entering the glass; once upon exiting the glass. If it hits both of those surfaces at the same angle, then it will continue in the original direction (although perhaps offset slightly) and there will be no noticeable effect. But when that line is striking near one of the 16 curves, the light will hit the inside surface at a slightly different angle than the outside surface - causing the path to be deflected by some angle.

Those 16 curves are along the equator. If you take an equatorial section of that green glass, you will not get a circle. You will get a smoothed version of a 16-sided polygon. It is those 16 bends, each stretching from the top of the glass to the bottom, that are causing the stripes.

What shape are the 16 curves? They are just a depression on the inside so I am assuming I can treat them as plano concave lenses?

 

[Zdravko]

I am an experimental guy :) . Try now to cover different parts of the green glass to se from which portion the most of the reflections on the floor are coming. I suppose that that will be from the part "south" of the equator. How does this part look like?

 

[Zdravko]

Looking now again at your first pic. it seems that dark areas are in alignment with thicker portions of the green colored glass. Which means that this portion is simply absorbing more of the light, and not acting as a lens.

 

[sophiecentaur]

This link tell you what you may want to know about fresnel lenses.
The lighthouse pictures show the lens but there is an equally important reflector, which rotates with the lamp shutter. This will be cylindrical, to make sure as much light as possible is directed onto each fresnel lens strip. The lighthouse system is more complicated than most fresnel lens systems.