Wednesday, April 23, 2014

Light Tricks

Vero Beach, Florida

Look at the picture below. It shows one of two gimballed oil lamps we have on board, and the light patterns on the wall when the sun streams through the portholes and the lamp's globe.   We see the same patterns with both lamps and with light coming from any one of six portholes.

As you can see, the pattern is dominated by horizontal lines.  No such lines are visible in the glass.  Both oil lamps make similar patterns.  I've tried cleaning the globes with vinegar to see if it might be dirt on the glass causing the lines.  Nope.  

The porthole glass is tempered.  I suppose it is possible that the light coming through them is polarized.  But I can't see how that plays any role.  Besides, the different portholes would have random polarization orientations, but the lines are horizontal no matter which porthole the light comes in.

You can see in the picture that light from the portholes that does not go through the lamp globe has no pattern.

I just noticed that the picture shows something that is not visible to my naked eye.  I see spider-web like connections between the lamp and the wall.  It looks like the light rays cross forming an X pattern in the air.  Hmm, very interesting but I still don't see how that makes the lines.

Could it be constructive/destructive interference as light rays reach the wall via two different paths?

To me, this is an unexplained mystery.  Why the lines in the light pattern? Is this something a PHD candidate could write a thesis on?  I could spend days staring at that pattern trying to understand it.  What do readers say?  Please comment.


  1. I believe the lines are too irregular to be diffraction/interference patterns. It looks to me as though the glass is of varying thicknesses as is typical of older glass. The differences may be great enough to be felt, but probably not. As the glass gets thicker, less light is transmitted (or, because of lens-like effects, scattered differently) and the "band" appears darker. some of the darkness might also be caused by minute sot accumulations on the minuscule edges of the regions of differing thickness. Washing the chimney in alcohol or an alcohol/soap mixture, followed by rinsing with distilled water should remove any soot.

    It's interesting that the image of the lower-half of the chimney (outside the direct light from the porthole) doesn't appear to have the lines (tho the contrast may be so low in the photo that they don't show up well). Are they, in fact, there?

    Interesting physics problem!

  2. Your engineer is showing. Sometimes it is better to just enjoy. Spoken by my wife many times. We need a materials scientist to chime in. Ken

  3. One might start with the fact that the glass of the globe is going to change the angle of the light waves as they enter the glass and exit the glass twice, once on each side. Surely the phase of the light waves are going to be distorted and thus there will be cancellation of the light waves in portions of the image projected on the bulkhead.

    Surely the science is at least related to prisms and diffraction gratings.

  4. Reader Dennis says,

    My astute observations as to “WHY” relates to the way the globes were likely manufactured.
    It is likely they are spun or blown, with the glass composition being not necessarily a fully homogeneous mixture,
    Rather more of en extruded type consistency.

    The pattern is due to the variations in the glass internal micro-crystallizations and refraction from that variance.

    Like your challenging questions and musings.
    Keep up the good work…

  5. Interesting image for sure - have you tried adding other lens in front of the light source to see what happens - of course we'll need a full photo essay to examine!!



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