Skulls in the Stars: Infinity is weird… even in infinity mirrors!
Even very simple optics can reveal very interesting and surprising phenomena, if one looks carefully enough! I was recently looking into the optics of a so-called “infinity mirror”, which in its simplest incarnation is simply two parallel mirrors on opposite sides of a room or elevator. The result is a multiplication of images, seemingly stretching out to infinity
Dot Physics: Can You Charge Your Phone by Typing?
The feasibility of charging a phone by typing, and by other means.
Uncertain Principles: How Good Are Polarized Sunglasses?
The procedure is simple: the laser passes through the G-T polarizer, which establishes a linear polarization for the beam, then it goes through the sunglasses lens, which transmits a fraction of the light that depends on the angle between its polarization axis and the G-T polarizer. The figure you use to measure the quality of the polarizer is the “extinction ratio,” which is the ratio of the minimum and maximum transmitted intensities. For an ideal polarizer, this would be zero– perfectly polarized light would be completely blocked by an ideal polarizer at 90 degrees from the light polarization– but nothing is perfect, so there’s always a little bit of light leaking through.
I was on a mini-vacation this past weekend, escaping the DC heat by going to a place that was only slightly less miserable (~95ºF instead of 100ºF) but I did have the benefit of a pool in which to do a little physics demonstration.
That’s me (I’m the one in the “Make it look like an accident”/Humpty was pushed t-shirt) waterboarding my camera. I set my GOPRO (in its waterproof case) to the time-lapse setting rather than the timer so it would take the pictures automatically as I immersed and panned it, and I wouldn’t have to keep disturbing the water’s surface to reset anything.
What you are seeing here is an effect called Snell’s Circle or Snell’s Window: the light from outside the pool refracts as it enters, and this bending means that the entire view of the outside is compressed into a cone.
Snell’s law tells us that
\(n_1sintheta_1 = n_2sintheta_2\)
Water has an index of refraction of 4/3, so light with a grazing incidence to the water (n=1 for air, and sin(90º)=1) will be bent to about 38.5º — arcsin(3/4) — with respect to the normal (perpendicular to the surface) so it acts as a wide-angle lens with a 97º angle into the camera. Outside of that circle what you are seeing is light from inside the pool undergoing total internal reflection — the camera is immersed only about 10 cm or so, and the blue you are seeing is from the sides and bottom of the pool.
The Astonishing Stroboscope of Doc Edgerton
When set to the right speed, the strobe appears to stop the subject in its tracks- a useful illusion. Deviations from stationarity indicate an object is spinning faster or slower than the strobe. Thus, the strobe is also a tachometer.
Whistle While You Work? Not in Space
Former astronaut Dan Barry has seven hours of spacewalking time to his credit. He tried whistling during his spacewalk on STS-96 in May 1999.
“It wasn’t something I hadn’t planned — I thought of it on the fly. It turned out that it didn’t work.,” he said.
Barry called down to Mission Control and said, “Houston, EV2. The science types might like to know that it is not possible to whistle during an EVA.”
This just in: many living (and formerly living) things contain carbon and hydrogen. Details at The Naked Scientists
You have to do this with a rotating system because of space considerations and I think the globe is really neat, but …
I have to pick nits here. I really wish the video makers/interviewer had drawn a better distinction between the granite globe and the earth. The globe in the video was certainly not created spinning as it is, and we know there is friction, so asking why it’s spinning (or not stopping, at about the 1:00 mark) is really the wrong question if you are going to discount answers that say it’s because there’s a force (or torque) on it. I’m not sure if I wouldn’t interpret “it’s not stopping” as meaning that it’s simply not coming to a stop while I’m watching it, rather than it will not come to a stop over a much longer period of time. A key in all this is the identification of friction as a force and not some inherent property — that sometimes requires some mental untangling. You have to be sure you’ve done that for a successful misconceptionectomy.
Plus, we know that the earth is slowing down as it trades angular momentum with the moon.