First time poster.

I’ve been reading Feynman’s Lectures on Physics, and I’ve just finished reading his treatment on diffraction. Feynman described a method of thinking about an opaque wall that was new to me (or, at very least, I don’t remember it). I’m looking specially at volume 1, chapter 31-6. Feynman describes the field on the dark side of the wall as being a superposition of a field due to the source and a field due to the wall. These fields exactly cancel to make the dark-side of the wall dark. (I’m used to thinking about the effect of an opaque wall in terms of photon absorption.)

I think I understand this way of viewing the effect in steady-state, but the model seems to breakdown when we consider transients.

If we consider a situation where an opaque wall is blocking the light from a source, then the field at some point on the dark side is this superposition of the field due to the source and the field due to the electrons in the wall. The electrons in the wall are being driven by source field, and the previous sections in Feynman’s book lead me to think we can consider them as damped oscillators. For a thick enough wall, the field due to the source will destructively interfere with the field from the wall until the net field is zero.

But, if I turn off the source, the electrons in the wall don’t instantly stop oscillating. The oscillating should die off with some time constant proportional to some material factors: whatever gives rise to the spring and damping constants. In the absence of the source field, shouldn’t we no longer have destructive interference on the dark side of the opaque wall?

Now, I understand the caveats that the information about source being switched off only propagates at c, and also that electrons deeper in the wall may start to become driven oscillators by the field produced by still oscillating electrons after the source is switched off.

However, that all being said, I think I can construct a scenario where (if this model is the right way to think about it) I get a flash of light on the dark side of the opaque wall when I turn the source on or off. Most simply, just consider an opaque wall that is one mono- layer thicker than what is necessary to completely block the light from the source. I have a hard time understanding how, in that situation, we would not get transient transmission when there are changes in the source. (However, it’s not at all clear to me this is the only situation where transients might be important, and I’ve not tried to go through the math to see how the transient might propagate in a thicker wall.)

This would give rise to flashes of light appearing to be emitted by the wall on the (previously) dark side when I shut off a source. Something similar ought to happen when I turn the source on, adjust it’s intensity or wavelength.

I’m unfamiliar with this phenomenon. Does it happen? Or is there a flaw in my understanding?

  • $\begingroup$ It seems to me that your reasoning is correct and therefore such a flash is possible, though I guess weak and hard to detect. $\endgroup$ – Andrew Steane Jan 5 at 11:28

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