It reminds me of twenny years ago when CDs were new, an audiofile friend of mine bought a "special pen" with which you painted the edge of CDs green. It stopped the laser light from being reflected back and messing up the playing. Problem is that even if we assume that that would really be a problem, then painting on the edge would not have any effect, since it would not stop light being reflected. The light is reflected by the phase change and won't care if it's a plastic/air surface or a plastic/paint surface.
Oooh, reminds me of a logical error in an otherwise very enjoyable SF novel, Consider Phlebas. The protagonists are shooting with laser guns at enemies hiding in an old ruin which is overgrown with moss. And they are being shot back at with lasers, at a furious rate. But then they discover that there is nobody in there, only the ruin is built from crystal, and the lasers are their own shots, bouncing around inside all the crystal and then being sent back.Sounds neat, almost a pity to ruin it with logic: if the laser shot had the power to break through the moss the first time, why did it stop doing it on the other side? Why not just go straight through? Also, not being mirror, the crystal would only reflect less than ten percent of the light, so after a couple of bounces it would have been reduced to nothing. And finally, after bouncing about in there, what would happen to make the beam suddenly decide not to do it anymore, but to burn through the moss suddenly and escape? If it could, wouldn't it have done it the first time?
Apart from that, a romping good action space opera book.
Lemme get this straight... you're arguing about the accuracy of a scenario in a sci-fi novel? Sheesh. That's like arguing in favour of the literal accuracy of the bible.
ReplyDeleteHaving said that...
Consider a pentaprism from one of your older SLRs. It refracts the light from the mirror behind the lens with less light loss than the modern "penta-mirror" setups - and it has no silvered surfaces - it just relies on refraction.
So, the scenario you described is possible... highly unlikely, but possible.
Jeff R.
I'll further add that the builders of the now-in-ruins city might have planned this cleverly hidden defense. The city could have been designed to shoot back at assailants! ;-D
ReplyDeleteI admit I never understood how a pentaprism reflects light. There must be some special coating only on some sides, otherwise the light wouldn't get in or out.
ReplyDeleteAha, I was right:
ReplyDeletehttp://en.wikipedia.org/wiki/Pentaprism
It is a mirror coating on two surfaces.
Now the mystery only remains how this mirror coating is more effective than a penta-mirror. Wait, I think I know: It's the reflections in the glass surfaces which take away from the latter.
Sighhhh...
ReplyDeleteOK
(Can't believe I'm arguing sci-fi here)
My bad analogy. SLR pentaprisms have mirrored surfaces. Glass (or crystal!) prisms don't need silvering to achieve close-to-perfect "reflection" of light, (it's refraction), so the sci-fi crystal return-the-laser-beam scenario is perfectly feasible.
Hold a glass prism to your eye, and you'll be looking through it to your right (or left). No mirrors, nothing up my sleeve.
I think a pentaprism *is* solid glass, it's what differentiates it from a mirror prism.
ReplyDeleteStill doesn't make sense to me. If *all* the sides have near-perfect reflection, how does any light come out to your eye?
"Still doesn't make sense to me. If *all* the sides have near-perfect reflection, how does any light come out to your eye?"
ReplyDeleteThere is an angle called the critical angle, the value of which depends on the relative densities of the two media. In the case of a prism in air, the denser the optical glass, the steeper the angle. If you were to put this same prism in water, the value of the critical angle would be less.
This angle is related to a phenomenon called total internal reflection. Light traveling from the high density medium towards the low density medium and striking the interface at or near the normal (e.g. 90 deg.) will pass the interface (i.e. exit the prism), while light striking the interface at an angle less than the critical angle will be completely reflected. That is why, for example, optical fiber is able to guide light over long distances even if bent (but bend radii can't be very sharp).
OK, I was ignorant of this phenomenon.
ReplyDeleteThis isn't as far-fetched as it may seem. I'm happy to see I'm not the only one to have studied refraction and optics in school.
ReplyDeleteI must only correct a slight detail in Bert's explanation: light arriving to the interface in the direction of the lower OPTICAL density medium (it's very different from density of mass), at or near the normal, will pass it IN PART. Another part of the light beam is always reflected by the interface, while a last, usually tiny part is absorbed. The more you move away from the normal and come close to the critical angle, the bigger proportion is reflected and less is refracted while passing through.
But I can easily understand how someone who hasn't studied this would remain completely oblivious to some admittedly curious phenomenons.
Still, I'm pretty sure that painting the edge of a CD is utterly pointless. That zone has very little hope of influencing the laser beam's behavior, precisely because said beam is perpendicular (="normal") to the disc surface. And so tiny, therefore localized. The edge is equivalent to fearing that the windows of a factory 300 feet away might dazzle a policeman at night by reflecting the light of his flashlight when it's not aimed straight at them.