Physics

Sun - January 13, 2008

Cosmology, Mathematics, Truth, Etc.

A recent radio interview with physicist and novelist Janna Levin delves into interesting stuff.

Krista Tippett's public-radio program Speaking of Faith presents wide-ranging interviews that I've often found interesting and moving. The latest program scarcely touches on religion, but is a discussion with theoretical physicist Janna Levin, author of the novel A Madman Dreams of Turing Machines. The novel is a fictionalized account of the lives of the 20th-century mathematicians Kurt Gödel and Alan Turing. I've long known of these men, because their work was important to the foundations of computer science, but Levin's book goes beyond their mathematics into their personal lives (and their disturbing deaths).

This is a fine program that presents philosophical and scientific questions dealing with cosmology, mathematics, truth, belief, intuition, purpose and free will, among other things. (There is some kind of time warp that allows all of this to fit into the 53-minute program. The unedited interview fits in even more stuff. Both are available for download, and both are worth listening to.)

I think I'm going to have to read the book.

Posted at 03:11 PM Read More

Wed - June 13, 2007

Remembering Mr. Wizard

Don Herbert--known to millions as "Mr. Wizard"--died yesterday.

I've been a science geek for the last half century. I don't remember how or when my science obsession began, though I recall that when I was in second grade (in 1956) I was envious of my older sister, because her third-grade class actually included science in the curriculum, and she had a real science textbook!

By that time, Don Herbert's TV show, Watch Mr. Wizard, had already been around for a few years. I can't be sure, but I'd guess I was watching the program by 1956. My personal copy of his book, shown in the photo above, has a 1953 printing date, but I don't think I was reading much at age 3. In any case, the show was certainly an inspiration for me for many years. He wasn't as zany as successors such as Bill Nye, but his experiments were often dramatic, and many could be readily reproduced at home by young viewers. He did a fine job of demonstrating a wide range of scientific phenomena and principles, explaining them in an understandable way without dumbing things down. A typical program showed him demonstrating and explaining his experiments to a young visitor (a boy or girl maybe ten or twelve years old) on a set that looked to me like a large kitchen. The set, the young participant, and his lucid explanations made everything seem quite accessible.

I give Don Herbert a lot of credit for inspiring and supporting my interest in science, and I'm sure he had the same effect on many of my generation.

Posted at 09:13 PM Read More

Sun - January 28, 2007

Blue LED Fluorescence

Blue light-emitting diodes (LEDs) cause fluorescence in common materials. Jim found this surprising; maybe you'll find it interesting.

Fluorescence is a phenomenon in which a material absorbs light of one wavelength, and emits light of a longer wavelength. It is often demonstrated using ultraviolet light, which has shorter wavelengths than visible light. Ultraviolet light causes many materials to emit visible light of various colors.

Fluorescent dyes and pigments are used to make many objects with bright colors (often referred to as day-glo colors, though the term is technically a trademark). I had long assumed that these materials relied on the presence of ambient ultraviolet light, but a few months ago, while using a small blue-LED flashlight, I was surprised to see a pen glowing bright pink in its beam.

I soon found that other blue LEDs I tried were also capable of making common fluorescent materials give off colors other than blue. The effect can be dramatic. If you shine a blue LED around a dark room, the scene is generally monochromatic, with objects ranging in apparent color from light blue to black. But here and there you may see something green or yellow or pink standing in stark contrast to its blue surroundings.

The photos below show the effect. Each shows a set of non-fluorescent colored pencils on the left, and parts of four fluorescent index cards on the right. The first photo was taken in daylight, and the normal colors of both the pencils and the cards can be seen (yes, the violet pencil on top did actually look nearly black, and the red pencil on the bottom was a rather dull red).

The second photo was taken in the light of a blue LED. The non-fluorescent pencils show very little color other than blue. The fluorescent cards look somewhat bluer than in daylight, but they clearly show varying degrees of green, orange and red hues as well. (To the eye, the cards seem much closer to their daylight colors than in this photo, but this may be due to the visual system partially compensating for the blue light.)

In fact, I've learned that this fluorescence is used to make white LEDs. LEDs generally emit a rather narrow range of wavelengths (which typically gives them very intense color). It would be possible to create white-looking light by combining blue, green and red LEDs, but most white LEDs are made from a blue LED combined with a fluorescent material that emits a range of wavelengths centered in the yellow part of the spectrum. The result appears white, but is often weak in red and green wavelengths, so colors can look unnatural in this light.

Posted at 02:27 PM Read More