Toothbrush, glasses, and clock

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ThirdParty
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Toothbrush, glasses, and clock

Postby ThirdParty » Sat Mar 25, 2017 5:15 am UTC

I've been inspired by the recent moiré comic to ask a question that I've been wondering about for a couple of years:

I have an electric toothbrush, a pair of eyeglasses which correct for nearsightedness, and a digital alarm clock-radio. It is possible to use any two of these three items simultaneously without incident. However, if I attempt to use all three at once--i.e. to brush my teeth (particularly my upper row of teeth) while wearing the glasses and looking at the clock--the numbers displayed by the clock start to dance cartoonishly, as if I were viewing them in a funhouse mirror. (Like, the bottom half of a number rises up while the top half compresses, and then the bottom half sinks while the top half stretches, and meanwhile the next number is doing the opposite.) The pace of the dance varies depending on which tooth I am brushing: it can be so fast that the numbers simply blur, or so slow that I think I'm imagining the numbers' gradual distortion.

Can somebody please explain this phenomenon? I presume that the clock is emitting polarized light (possibly flickering polarized light, but if so the flicker is too fast for my eyes to normally detect) and that the toothbrush is causing my glasses to vibrate, but I'm not clear about why polarized light plus vibrating lenses would create a funhouse-mirror effect.

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Re: Toothbrush, glasses, and clock

Postby DavidSh » Sat Mar 25, 2017 2:16 pm UTC

This is completely idle speculation, but perhaps the image on the display is only showing a line at a time, and depends on persistence of vision to form an entire image? In that case, vibration of the eye might cause different parts of the image to be displaced differently. I'm not sure how would check for this, though. Maybe photograph the display with a shutter speed less than 1/60 seconds.

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Re: Toothbrush, glasses, and clock

Postby ThirdParty » Sat Mar 25, 2017 2:40 pm UTC

I doubt it. The clock's display looks like a standard numeric LCD that composes each number out of seven line segments; it's not a dot matrix.

I don't have a fancy camera available, but I tried taking a picture of the clock with my cell phone. It looks normal in the picture. I also tried applying the toothbrush to the cell phone while taking the picture. Still normal.

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Re: Toothbrush, glasses, and clock

Postby Sableagle » Sat Mar 25, 2017 2:55 pm UTC

Try looking at other things through your glasses while brushing your teeth. Divide the world into things that do and things that don't look different that way.

Also, try looking at a Nissan's windscreen, because those show really pretty rainbow patterns when viewed through polarised sunglasses.
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Re: Toothbrush, glasses, and clock

Postby Soupspoon » Sat Mar 25, 2017 3:20 pm UTC

Sableagle wrote:Also, try looking at a Nissan's windscreen, because those show really pretty rainbow patterns when viewed through polarised sunglasses.
Just Nissan's? It's been a long time since I had clip-on lenses (polarised), rather than the second pair of specs of tinted lenses (no polarising), but I'm sure it was a feature of almost all car windows to have patterns (presumably from the tempered and/or laminated nature of the glass).

(Sometimes I can still see the effect if looking at a car windscreen through a tinted bus window, perhaps indicating that the bus's tinting is also polarising. I presume for better proportional performance in low-sun daylight than at night time, or something, because it would be an unnecessary fuss to do it with a polarising applique 'just because'.)


To the original problem (I nearly forgot to add), in a dark room with a bright item (computer or tablet screen) and dawn/dusk-lit window shades, moving one's head produces a lag of perception of the two bright spots, according to the relative luminosities, against the common dark background, indicating a perceptual responsiveness in some way keyed to the brightness of the ocular input. Depending on the angle of viewing the clock display, could each extremis of the display be significantly different and, added to the small but significant mechanical oscillation of the head, produce the effect described?

(Late additional note, also nearly not mentioned: I am myopic, mysef, to the extent that I need my glasses even to properly read a tablet display held naturally at 'forearm length', so I tend to forget that I'm wearing specs, only to realise when I'm not. While there's an obvious vertical compression of items if I tilt my lenses (keeping my head still, raising the legs/arms/whatever off of my ears), it's not something I've tested for (nor expect with) differential effects w.r.t. illumination levels, or other lagging/leading effects. This evening I shall try the above both with and without my visual aids, though, and at least a subjective comparison.)
Last edited by Soupspoon on Sat Mar 25, 2017 4:30 pm UTC, edited 1 time in total.

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Re: Toothbrush, glasses, and clock

Postby morriswalters » Sat Mar 25, 2017 4:07 pm UTC

How many of the possible combinations are relevant? I see two. Brushing with glasses on and with them off. I'm assuming the OP is brushing while looking at the clock. I have two questions. Is the toothbrush an orbital brush or does it reciprocate around the long axis? And does the clock have a segmented display?

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Re: Toothbrush, glasses, and clock

Postby ThirdParty » Sat Mar 25, 2017 4:43 pm UTC

Sableagle wrote:Try looking at other things through your glasses while brushing your teeth. Divide the world into things that do and things that don't look different that way.
A second copy of the alarm clock (they were on sale X years ago, so I bought two) also displays the phenomenon.

Nothing else in my house displays the phenomenon. (That includes digital clocks on two kitchen appliances, as well as a digital wristwatch.)

I found some non-prescription sunglasses from an optometrist appointment. They're not clip-ons, but they're meant to be worn over my regular glasses. I suspect that they're tinted rather than polarized; they make everything look green. The effect still occurs if I wear the sunglasses over my regular glasses. The effect does not occur if I wear just the sunglasses and not my regular glasses. (The effect does still occur if I substitute my previous pair of glasses, that had a slightly weaker prescription.)

Soupspoon wrote:Depending on the angle of viewing the clock display, could each extremis of the display be significantly different and, added to the small but significant mechanical oscillation of the head, produce the effect described?
The effect seems most pronounced when I view the display directly head-on. It dwindles to essentially nothing if I view the clock from a sharp angle.

If I view the clock head-on but turn my head sideways, the angle at which the numbers dance changes. Instead of getting taller and then shorter, the get wider and then narrower. (In the other words, the movement is always along my head's up-down axis, not the clock's up-down axis.) :!:

morriswalters wrote:How many of the possible combinations are relevant? I see two. Brushing with glasses on and with them off. I'm assuming the OP is brushing while looking at the clock. I have two questions. Is the toothbrush an orbital brush or does it reciprocate around the long axis? And does the clock have a segmented display?
The toothbrush looks like this. It has one component that rotates and a second component that oscillates side-to-side.

As far as I can tell from touching only part of the toothbrush to my teeth, both components are capable of generating the effect. The effect produced by the oscillating component is more pronounced if I align the oscillation with my head's vertical axis.

The clock looks like this. Standard numerical display.

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Re: Toothbrush, glasses, and clock

Postby Sizik » Sat Mar 25, 2017 5:18 pm UTC

I think this explains the phenomenon.
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Re: Toothbrush, glasses, and clock

Postby Tub » Sat Mar 25, 2017 10:41 pm UTC

ThirdParty wrote:I also tried applying the toothbrush to the cell phone while taking the picture. Still normal.

Only a true scientist would apply a toothbrush to a cell phone and consider that process "normal". Well done.

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Re: Toothbrush, glasses, and clock

Postby Sableagle » Sat Mar 25, 2017 10:50 pm UTC

Tub wrote:
ThirdParty wrote:I also tried applying the toothbrush to the cell phone while taking the picture. Still normal.

Only a true scientist would apply a toothbrush to a cell phone and consider that process "normal". Well done.

You don't use your old toothbrush on your cell phone? :shock: How do you de-scratch the screen?
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Re: Toothbrush, glasses, and clock

Postby Eebster the Great » Mon Mar 27, 2017 4:35 am UTC

I've observed the same thing. It is extremely pronounced for certain LCD clocks. It can actually happen even when my glasses are off, but it is an order of magnitude less significant. I think the glasses are relevant because the comparatively subtle vibration of my face makes my glasses move a longer distance (so less acceleration but greater average speed), but I'm not certain.

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Re: Toothbrush, glasses, and clock

Postby ConMan » Mon Mar 27, 2017 4:59 am UTC

I've seen a related effect - most things look normal when I use an electric toothbrush with the room lights on, but if the room lights are off, then bright objects show a distinct vibration to them. I think that when the lights are on my brain has enough information to do a kind of image stabilisation, but when there's only one bright thing, it goes with what it sees, which is the bright thing moving back and forth.
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Re: Toothbrush, glasses, and clock

Postby ThirdParty » Mon Mar 27, 2017 12:40 pm UTC

Sizik wrote:I think this explains the phenomenon.
Aha! Yes, that's helpful. I had no idea that clock displays cycle each segment rather than just having them all on at once, but it makes sense that you wouldn't want to risk an "8" being dimmer than a "1".

Eebster the Great wrote:It can actually happen even when my glasses are off, but it is an order of magnitude less significant.
Definitely possible. I'm sufficiently blind without my glasses that I wouldn't notice a subtle effect.

Eebster the Great wrote:I think the glasses are relevant because the comparatively subtle vibration of my face makes my glasses move a longer distance (so less acceleration but greater average speed), but I'm not certain.
That sounds plausible.

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Re: Toothbrush, glasses, and clock

Postby plytho » Mon Mar 27, 2017 12:55 pm UTC

I've noticed a similar thing. When driving at night LED headlights dance around when viewed in my rear view mirror. No other headlights, only LED.
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Re: Toothbrush, glasses, and clock

Postby Soupspoon » Mon Mar 27, 2017 1:29 pm UTC

(LED headlights are probably creating more dangers than they are overcoming. Like letting people drive around in indestructium shells, and bugger anyone they'll 'safely' crash into.)

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Re: Toothbrush, glasses, and clock

Postby gmalivuk » Mon Mar 27, 2017 1:56 pm UTC

Eebster the Great wrote:I've observed the same thing. It is extremely pronounced for certain LCD clocks. It can actually happen even when my glasses are off, but it is an order of magnitude less significant. I think the glasses are relevant because the comparatively subtle vibration of my face makes my glasses move a longer distance (so less acceleration but greater average speed), but I'm not certain.
I suspect it's more to do with the fact that even small movements of your glasses change the angle of the things you're seeing, and not in a way your brain automatically adjusts for like it would if it were just your head movements.
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