12 Mar 2009, 7:21 pm
robo37 wrote:
PhaethonH wrote:
Colour is perceived based on how much each receptor is stimulated. For example, violet is perceived when the red and blue receptors are stimulated by about the same amount. There are actually two ways of accomplishing this:
- combine a red light (650 nm) and a blue light (475 nm) in about equal proportions, stimulating each receptor type separately at the same time.
- use a pure violet (400nm source) light, which then happens to stimulate the red and blue receptors by about the same amount together at the same time.
My physics teacher said that if you send any type of violet light through a red coloured filter it always come out as red light.........how would that be possible without any red light to start with? And about that 'light always takes the shortest path ' thing, another physics teacher drew a diagram of a hot day, where light is coming straight from the sky towards the ground but then bends towards some guys eyes just before it touches it..........wouldn't it be quicker for the light to just go in a straight line?
Oh yeah thanks for the help BTW.
The path light takes will be bent as it passes from a medium of one index of refraction to another. This is how refraction works. If light moved through a medium of continuously varying index of refraction, it's path would bend.
12 Mar 2009, 9:55 pm
robo37 wrote:
PhaethonH wrote:
Colour is perceived based on how much each receptor is stimulated. For example, violet is perceived when the red and blue receptors are stimulated by about the same amount. There are actually two ways of accomplishing this:
- combine a red light (650 nm) and a blue light (475 nm) in about equal proportions, stimulating each receptor type separately at the same time.
- use a pure violet (400nm source) light, which then happens to stimulate the red and blue receptors by about the same amount together at the same time.
My physics teacher said that if you send any type of violet light through a red coloured filter it always come out as red light.........how would that be possible without any red light to start with? And about that 'light always takes the shortest path ' thing, another physics teacher drew a diagram of a hot day, where light is coming straight from the sky towards the ground but then bends towards some guys eyes just before it touches it..........wouldn't it be quicker for the light to just go in a straight line?
Oh yeah thanks for the help BTW.
= Of Violet Light And Red Filter, or I Have No Idea What I'm Talking About Here =
OK, that thing about any violet light through a red filter is something I haven't dealt with before, so I'm making a shot in the dark (teehee) here...
One way of modeling filters is to describe them as "blocking any colour light except its own", such that a red filter blocks all but red light.
Another way of modeling filters is as a mass of atoms that absorb light energy on one side, and preferentially transmit a particular wavelength out the other, based on the "jumping electrons" principle. Light energy of the "wrong colour" would fail to energize the electrons (or partially energize). In this model, a red filter consists of atoms that all have "red-sized" jumping electrons, light coming in one side energize the electrons to "jump up", and when they "jump down", they spit out red light on (mostly) the other side.
Right now, I want to say that the violet light partially energizes these red-energy electrons, so that red light comes out the other side. The problem I have with this explanation is then why green would still appear black through the filter, even with a wavelength closer to red than violet is. At which point I want to say something about frequency resonances, but the ratios are all wacky and doesn't make sense.
Nonetheless, AFAIK, a "true" violet light (400 nm coherent) cannot pass through a "true" red filter (650 nm filter). But both are difficult to obtain in "pure" form.
The far simpler hypothesis is that "most violet light" is really "mostly-violet" along with reds and blues such that the light appears violet to the human eye -- there you get the red that makes it through the filter. The most reliable source of a "true" violet light would be a violet laser, followed by a full-spectrum ("white") light source split through a prism.
Another plausible means of getting red from violet is to pass the violet and add an "anti-blue" to the violet, in this case, yellow. So maybe the filter is also emitting yellow light with violet when struck by violet? This hypothesis would be easily testable with a prism (but probably hard to see with the light getting dim through all those obstacles).
Do I smell an experiment coming up?
= On Refraction, or Why Straight Does Not Mean What You Think it Means =
As for straight lines, the basic summary is "the meaning of 'straight' changes". Consider this, an overhead diagram of crossing a sandbox:
Code:
## A ########################################## <- start here at 'A'
############################################### <- concrete surface
::::::::::::::::::::::::::::::::::::::::::::::: <- sandbox
:::::::::::::::::::::::::::::::::::::::::::::::
############################################### <- more concrete
########################################### B # <- end here at 'B'
############################################### <- concrete surface
::::::::::::::::::::::::::::::::::::::::::::::: <- sandbox
:::::::::::::::::::::::::::::::::::::::::::::::
############################################### <- more concrete
########################################### B # <- end here at 'B'
For most people, running on concrete is much easier than running through sand. Is the quickest path from A to B formed by drawing a straight line on the map? Consider how much time you're struggling through the sand if you ran on a straight projection from A to B. Now consider running towards the half-way mark of the sandbox (aiming a little more to the runner's left), then turning right to wade "straight" down the sand box, then turning left again as you hit concrete towards B. Less distance through sand means less time being slowed down by the sand.
The point I'm trying to get across here is that the less time you can spend in the sand, the faster overall time you can achieve. And the path that spends the least amount of time in the sand is not along the straight line you draw from A to B, but some crooked monstrosity. However, the "straight-down" path through the sand means you're spending more time (too much time) pounding concrete. The "sweet spot" in between is a little to the left at the start on concrete, then a little to the right through the sand, then back to left when on concrete again. This path is not a straight line when drawn on the map, but it is the quickest path.
The "sandbox" trap is analogous to what the path of light faces in changing densities. The sandbox represents the denser medium, as denser substances slow down light more significantly. (In the presence of gravitational wells (blackholes), "straight" is literally redefined)
Something else I want to point out as well is that in the case of light paths, straight is subjective, since the very means you use to determine if the path is straight (vision, light) is the very thing that's defining the path (light. vision). Note that drawings of light paths are models -- simplifications -- of events in real life, so you lose some information and you gain information that does not really exist. Access to these drawings are a benefit that real photons lack. Most importantly, in a static image, you lose information about how fast the photon is going at different points in the path -- this affects what the "quickest" path is. I don't know how much exposure you have had to the differing speeds of photons (not "speed of light", aka c), but the lack of this information may be skewing how you perceive the diagrams (again, information lost in simplifications).
Hope that helps some.
14 Mar 2009, 6:50 pm
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