EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: Logan on April 22, 2021, 12:09:57 pm
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Hello.
Just out of curiosity, does such material exist that only pass light one way?
Thanks.
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https://en.wikipedia.org/wiki/Optical_isolator
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No, to do so would require a variant on Maxwell's Dæmon (https://en.wikipedia.org/wiki/Maxwell's_demon) that reacted to the direction that a photon was travelling and hence would violate the laws of thermodynamics for reasons similar to why Maxwell invented his dæmon thought experiment.
Although optical isolators, as pointed out by Amyk above, exist they aren't a generalised one-way mirror. For a start, they don't differentially reflect light depending on its direction of travel, they just cancel it or shift it off axis.
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I think a "light-diode" can exist, and no imaginary "Photo Maxwell's Daemon" required. For example, directional couplers, circulators and isolators are curently used in most of the RF/microwave equipment to separate electromagnetic waves by their propagation direction (even the most ubiquitous RF device, the mobile phone, won't work without them).
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Now imagine one of those devices operating with signal levels that are the same amplitude and at the same frequency on both sides of them, and yet only allowing the signal from one side to flow unattenuated to the other, and signals flowing in the opposite direction to be reflected totally. That's what you're asking the putative one way mirror to do. Existing one way mirrors that rely on differential lighting partially reflect and partially admit.
Edit: Note that you can do this with light with coherent radiation (i.e. lasers) and that a lot of the RF techniques mentioned also rely on having coherent radiation (e.g. cancelling a signal by having a 180º phase shift).
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OK, but "totally", is never observed anyway in the real, physical reality.
Reality is always messy and uncertain, "ideal" is only a virtual concept, concept that can never exist in nature no matter how advanced a technology might be.
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Hello.
Just out of curiosity, does such material exist that only pass light one way?
Thanks.
As Cerebus says a one way mirror is not exactly a device which allows "light to pass only one way" - to function as a mirror on one side it must reflect at least some of the incident light, although for the typical purpose of observing without being observed a black screen one one side would actually suffice (but be even more obvious than a "one way mirror").
Of course these days, for the purposes of observing without being observed a regular mirror with a camera in the frame and an LCD screen the other side is probably easier, more flexible and produces superior results.