Naked Science Forum
General Science => General Science => Topic started by: Justin Gmail on 06/01/2010 18:30:02
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Justin Gmail asked the Naked Scientists:
Hi Dr. Chris and crew,
I love the show (http://www.thenakedscientists.com/HTML/podcasts/) and I've downloaded the entire catalogue in iTunes. I'm listening to the shows (http://www.thenakedscientists.com/HTML/podcasts/) on my commute in chronological order so please forgive me if my questions have been answered already -
It's simple enough to understand how a flashlight works, but would it be possible to create the opposite? A device that either projects shadow, or somehow removes light from it's path?
Thanks again and keep up the great work! I feel smarter for having listened to your show (http://www.thenakedscientists.com/HTML/podcasts/)!
Justin in New Hampshire, USA
What do you think?
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A "micro-black holes" gun? [;D]
I don't think such a thing you ask can exist (at least in the near future).
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To make an illuminated area appear darker an inverse flashlight would need to emit light capable of interacting destructively with the light reflecting from the illuminated surface. In other words, the light emitted from the torch would need to be of equivalent wavelengths and out of phase with the light from the surface; this is very difficult to achieve!
Chris
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Not a 'torch' type device as such, but there is a device, usually used in pairs, that makes rooms darker; they're called curtains [;)]
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Can't you just use one of these ? [;)]
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Not a 'torch' type device as such, but there is a device, usually used in pairs, that makes rooms darker; they're called curtains [;)]
An umbrella would be more directional.
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To make an illuminated area appear darker an inverse flashlight would need to emit light capable of interacting destructively with the light reflecting from the illuminated surface. In other words, the light emitted from the torch would need to be of equivalent wavelengths and out of phase with the light from the surface; this is very difficult to achieve!
Chris
Chris, that's not exact. In the best situation, that is with coherent beams of light, you would have destructive interference only every λ/2 metres (and constructive in between): two equal and opposite coherent beams cannot canceal, they make a standing wave; look for example, under the name "standing waves" at the picture up right here:
http://en.wikipedia.org/wiki/Standing_wave
Let's take visible light with λ = 600 nm (in the orange colour): then the destructive interference is in x1 = 0, x2 = 0.0003 mm, x3 = 0.0006 mm and so on, with constructive interference in x = 0.00015 mm, 0.00045 mm, 0.00075 mm ecc.
In other words: you won't see any difference.
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You know this is a very clever question, I can tell because it's giving me a headache again. Normal light from the sun, so called incoherent light, comes in a blend of all kinds of wavelength. The idea behind it I guess would be that every time the incoming light creates a crest your light will send out a trough that effectively would cancel out that crest?
There is a particle/wave duality to light and if you look at light as 'particles' like photons they shouldn't ever be able to interact, but seen as waves they seem to be able to do it. Then comes the question if you can do it meeting light, or if you must be traveling the same way.
Thinking of it I presume that the only way possible is the one meeting light as you can't catch up on light (If you see how I mean:) Light is already the fastest there is so you can't really catch up on light that already left, not even with light.
So okay, we know that it has to be waves that must meet other waves 'head on' so to speak, do you agree? And that every crest in that sun beam must be meet by a wave trough to create that 'flat line' that then would be the light extinguished.
So how many wavelengths (crests and trough's) do that sunbeam contain in 'one spot in time' that you need to extinguish simultaneously?
And it leads to more questions, assume that you could quench that light, does that really mean that the energy is gone, after all, you have given that spot in space where you quenched the light two times the energy when doing it. The incoming energy meeting the energy from your light, and then disappearing at that spot. But is it gone or do that spot now contain both energy's?
There is something called 'many paths' (Feynman) that suggests that light takes all paths it can simultaneously, but with different probabilities. The one most probable we call the lights 'real' path, but as we can't see light in itself but only its interactions we can't even be sure if light travels.
Everything you see is in some ways 'reflections' from lights 'interaction' with f ex. the screen you look on as you read this. That light hitting the screen will then change the wavelength of the light as it 'reflects' back, now as a 'new' photon, and then to your eye, where it will interact again with the rods and cones in your eye, creating a electrochemical impulse to your brain.
So what you think you see as light is then the interactions when photons meet 'matter' of different kind. Do you see what I mean? That under no circumstances can we observe 'light' in its pure state, only its interactions.
Lightarrow above discuss coherent light (lasers) and standing waves (http://www.colorado.edu/physics/2000/microwaves/standing_wave1.html) but even with those one you cant quench/extinguish light as far as I know?
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Not a 'torch' type device as such, but there is a device, usually used in pairs, that makes rooms darker; they're called curtains [;)]
An umbrella would be more directional.
They're not as good as curtains, at least from the keeping-light-out point of view though. Very good if it's raining, of course. Mind you, if rain is a problem inside your room then the lack of a roof is going to rather counter the efficacy of the curtains, so you may have a good point there.