Lightning Facts

Some Lightning Facts

A typical lightning flash lasts about a quarter of a second and consists of 3 or 4 individual discharges called strokes. Each stroke lasts a few ten thousandths of a second, although the visual appearance is longer. The “flicker” sometimes observed in lightning is due to seeing the actual strokes making up the flash.

A lightning stroke begins with a faint predischarge, called the leader, which goes from the cloud to the ground. The leader establishes a path for the highly luminous return stroke (what you really see) which propagates from the ground up to the cloud. The first stroke of a flash is usually preceded by a “stepped leader”, so called because it appears to progress in discrete steps (about 100 segments, each 50 m long) from cloud to ground. The subsequent strokes are preceded by a “dart leader” which smoothly follows the path of the previous return stroke (and is about 10 times faster).

Thunder is formed from the shock wave formed by the rapid heating of the air along the path of the return stroke, which reaches some 30,000 degrees K. The sound of thunder varies depending on how far you are from the various parts of the stroke. The sound from a part of the stroke farther away will get to you later and be fainter. {more}

What the Water Spider Taught Me

“What the Water Spider Taught Me”
(2002) The following are some very interesting optical effects that can be observed at the meeting of a gas, liquid and solid, manifested in surface tension and capillary effects, that seem to contradict what I have found on the subject in my Fluid Dynamics and Chemistry books. However, what is wonderful is that in these effects, the behavior of the transition state (liquid) between solid and gas, also indicates something about our soul conditions, and this is in some ways a reminder of the alchemical aspect of scientific work.

In the 12th grade physics, addressing light, color and epistemology, we begin with a trip to the lake. In the crisp autumn morning we walk in silence to Wonderland Lake and spend about ½ hour studying the lake and surroundings for visual phenomena. Edelglass, Meier, Davy, and Gebert (pg. 81) nicely describe this activity in the book, “The Marriage of Sense and Thought”. Customarily, students will readily notice the reflections in the lake of the surrounding trees and mountain landscape, they will notice the lifting of objects seen in the lake, they will notice ripple effects (and their interaction with the previous scenes). And there is also a very special, rich observation that had eluded me until a recent journey with my 12th grade. Having made all the previous observations before, I had the time to be fascinated by one new, strikingly beautiful observation–having to do with a water spider.

I would like to suggest that this moment with the water spider has led me to restructure and reexamine the whole of this course, and even indicates work to be done in 9th or 10th grade that may be pertinent, to help set the stage for this discussion in 12th.

First: On Reflections
Though I will generally skim over the more everyday observations (which are nonetheless very important and have been discussed in other writings), I would like to highlight several discussion points that are important for later use in the course.

Reflections are common observations, but the students should really make several key observations (perhaps with help). Remember that movement is important! This is partly what separates experiences in the world from those in the laboratory and helps develop a sense for holistic seeing. Children (think back to when you were one) often carry wonderful observations related to movement in the world. Students can think that the reflected image may be stretched vertically as compared with the tangible one, until they crouch (move vertically) down to the water level. From this viewpoint they will seem to be equally sized, or nearly so. The reflection will be disturbed and distorted as ripples move through it; they may even point out that in the disturbance are patterns of light and dark and ‘pieces’ of the sky appear or disappear. As they move sideways along the shore, they will notice that the reflected image remains always aligned between them and the object, constantly reorienting itself to their movements. This makes them a participant in the observed phenomenon. This sense of movement and smooth, constant adjustment will be crucial to later discussing the rainbow. Several students may comment on the relationship of the sun to the object, to its shadow, and the reflection. Notice that the shadow remains in place as I move, it is tied more closely to the tangible object, while the reflection is more related to me.

Where is the reflection? If it ‘exists’ on the surface, why do we need to focus as though it is farther away (¼ mile or so in the case of a tree at the opposite shore)? Focussing on a floating leaf, then focussing on the reflection of a tree, your body tells you that the tree is much further than the leaf (surface). You must “spread” your eyes to see the tree in focus–your eye musculature responds as it does to distant objects. Note that Escher has a nice print called “Three Worlds” in which all three visual spaces are drawn in focus–something that could never happen at the lake. Also, closer objects (my feet at the shore) are larger than farther ones, as the familiar laws of perspective indicate. And a reflected space is established, in fact, with all the laws of perspective we would expect, one which appears to be the same size as the tangible space it reflects, can in fact almost be visually indistinguishable from the tangible world, and also is intimately connected to me (observer) and the tangible object as verified through movement. Which is real? Is not the reflected space just as real? The tangible world has certain visual properties, material properties, chemical ones, etc. I expect that a tangible tree (by definition) is one, which I could reach out and feel with my hand. The reflected tree is just as real, but only possesses visual properties that are shared with and correspond to the familiar ones of the tangible world. My crouching friend next to me describes a similar but slightly different experience of the reflected space from me, standing up. However, as we move and continue to observe, we are able to sift out an understanding of reflection. Is there lawfulness? Can we characterize all these observations in the statement of a law? In class, we will penetrate some of these observations more fully with the “cups” demonstration (see Edelglass, pg.83), and derive the law of reflection.