## Archive for July, 2011

### Buzz Lightyear Physics

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

### Not Coming Soon: the iPhone Workout

Dot Physics: Can You Charge Your Phone by Typing?

The feasibility of charging a phone by typing, and by other means.

### Holy Multivalued Function!

The Batman equation

I have not verified this myself…

Update: in addition to the comment below, there is this at nOnoScience.

### ZZ Top Physics

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.

Bulwer-Lytton Fiction Contest 2011 Results

Words hoping to live up to the legacy of It was a dark and stormy night

Winner: Romance

As the dark and mysterious stranger approached, Angela bit her lip anxiously, hoping with every nerve, cell, and fiber of her being that this would be the one man who would understand—who would take her away from all this—and who would not just squeeze her boob and make a loud honking noise, as all the others had.

### Poolside Optics

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.

### Getting Strobey With It

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.