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Properties of Light

taras

taras

Least Haunted
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If light is particles and a wave, then how come on very windy days, light sources such as a traffic light at 20m don't appear dimmer? The particles would get blown around and disrupted...

I think it's quite obvious there's something I'm missing here, but I felt this question had to be answered!! :)
 
hummmm yessss... the problem with light is it 'apears' to be a wave in some measuring situations (but then waves can propergate tho a vacume as it does in space)..and a particul in others...(as in duel slit experiments)...it seems to be neither.. it seems to be mysterious and strange! Tho things do slow its speed down its top speed (thro a vacum) is the top speed of everything!...anything that causes a speed change causes defraction (a change in direction) so if it were say underwater it would be dimed...or if some crystals were interposed... light is a very odd thing rather like gravity (which aparently travels at the speed of light too!.)
 
Does gravity affect light? :confused:
 
ah one of the odd things ..yessss it does...hence black holes..where gravity is soo intense even light gets pulled in...and stars etc are efected..ie gravity curves the light so they arent where they ought to be...
 
taras said:
If light is particles and a wave, then how come on very windy days, light sources such as a traffic light at 20m don't appear dimmer? The particles would get blown around and disrupted...

I think it's quite obvious there's something I'm missing here, but I felt this question had to be answered!! :)
Click to expand...
In a medium such as air, light travels slower than it does in a vacuum. The refractive index of a material is the ratio between the speed of light in a vacuum and the speed of light in the medium, so for the glass that the "high index" thin spectacle lenses are made from (typically with a refractive index of ~1.8), the light has been slowed down by a factor of ~1.8 when compared with a vacuum.

This isn't an important effect in air, as the refractive index of air is pretty close to one, i.e. the speed of light is only slightly slower in air, than it is in a vacuum, i.e. ~300000000 m/s. Now as a fairly strong wind might be ~30 m/s (corresponding to ~110 km/h), this corresponds to a change of 1 part in 10000000, which as you might imagine isn't a lot. Even if we neglect some of the subtler relativistic effects , you can see that you shouldn't notice an change in the behaviour of light. :)
 
taras said:
Does gravity affect light? :confused:
Click to expand...
Yup! The slightly confusing thing here is that light always travels in "straight lines". Now you've probably spotted the problem here, "But thought that gravity makes light bend?" You'd be quite correct.

What you have to do is go back to what you mean by a "straight line"? The simplest way to define a straight line is to say that it is takes the shortest distance between two points. This works fine, if space is "flat" such as on a table top, but what happens if space is curved, such as on the surface of the earth. In this case, the shortest distance between two points is a "geodesic." (On the earth, these correspond to segments of a great circle, e.g. a line of longitude.)

Now that we've extended the notion of a straight line (e.g. that path that light follows) to a curved space, we can see how light bends around objects that "curve" space-time.

The trick now is to show that a gravitational mass causes space-time to deform, but that is a story for another day. ;)
 
Now being one that sells specs for a living I happen to know that I have one pair of glasses with a refractive index of 1.9 and another pair with an index of 1.67.
Does this mean that I will see things slower through the higher index.


(Just for info: normal lenses 1.5 or there abouts, thin lenses 1.6, ultrathin 1.67, super thin 1.7. These are all plastic. In glass 1.5 1.8 or 1.9)
 
As a side bar, the difference betwixt c and the speed of light in air is significant enough to result in visible Chernenkov (sp?) radiation if a particle is sufficently energised which has been achived and is not the result of ionisation.

For those that don't know, if a particle tries to exceed the speed of light in the medium, the excess energy is 'shed' as a cone of photons along the trajectory of the particle. The more the particle tries to exceed SoL in the medium, the longer the wave length of photon, hence a radioactive fuel rod causes a blue glow in water, due to high energy particles being emitted by radioactive decay. Of course the difference between c and SoL in water is almost an order of magnititude.

OT but I thought it may be of interest :)
 
The disruptive effect you are describing is very noticable when you try to observe stars through a windy atmosphere; it is called twinkling...

oh, and astronomers sometimes call the quality of the atmosphere, which depends on the movements of he air, particularly heat haze, the 'seeing'; as in
"the seeing is good tonight, I can see all seven sisters in the Pleiades.."

http://www.assa.org.au/observing/seeing/
 
Is it correct to say that when you are looking at the night sky (or at the sun) you are looking into the past?
 
Bannik said:
Is it correct to say that when you are looking at the night sky (or at the sun) you are looking into the past?
Click to expand...

Technically, yes. Eight minutes for the sun isn't it? Not sure about the moon.
 
Thanks. I thought that was the case but I never actually heard it put that way. Amazing.
 
Bannik said:
Thanks. I thought that was the case but I never actually heard it put that way. Amazing.
Click to expand...

Yup, I look up at stars and wonder what I would see on the earth if I was in the vicinity of a certain star with very powerful binoculars. If the star was millions of light years away, I could see the dinosaurs, if it was several hundred I could see the Picts and what they got up too. A bit closer and I could witness the rise of Hitler or even watch Led Zeppelin play live. Well, it fascinates me anyway!
 
Yuppers.

I'm working on the FTL and 'the super Physick long distance seeing thingy' constantly these days - much simpler than a time machine.


Cherenkov radiation - scary as f*ck.
 
You can get Chernenkov radiation in an atmosphere, I found this account on the Insultingly Stupid Movie Physics page:

"in May of 1946 Dr. Louis Slotin, a Manhattan Project scientist, died of severe radiation sickness caused by plutonium. Slotin was engaged in an experiment called tickling the dragon's tail in which two beryllium-coated plutonium hemispheres were brought together in close proximity. The combined mass of the two hemispheres equaled the critical mass required to initiate a fission chain-reaction; that is, if they touched. The idea was to come as close as possible without touching. Slotin was holding the top hemisphere with his left hand when the screw driver separating them slipped. The room was filled with a blue glow and a massive emission of neutrons."

http://www.intuitor.com/moviephysics/
Click link for Sum Of All Fears
 
Bannik said:
Is it correct to say that when you are looking at the night sky (or at the sun) you are looking into the past?
Click to expand...

Well everything you see is technically the past!
 
siriuss said:
Well everything you see is technically the past!
Click to expand...
Just extremely recent. For instance, at the moment I don't know if Picadilly Circus is still there, or it's vanished in a puff of ions in the last 1/30000000 of a second.

Nope still there. At least it was then. Don't know about now, though.
 
anome said:
Just extremely recent. For instance, at the moment I don't know if Picadilly Circus is still there, or it's vanished in a puff of ions in the last 1/30000000 of a second.

Nope still there. At least it was then. Don't know about now, though.
Click to expand...

Maybe it does disappear every 1/30000000 th of a second, you just don't notice.
 
Neither does the present if you think about it.
 
siriuss said:
Maybe it does disappear every 1/30000000 th of a second, you just don't notice.
Click to expand...
It'd flicker a bit.

Or at least a bit more. (All those neon sign - hate to have to pay their electric bill.)
 
Eburacum45 said:
The disruptive effect you are describing is very noticable when you try to observe stars through a windy atmosphere; it is called twinkling...
Click to expand...
Though, strictly speaking this effect is due to spatial and temporal fluctuations in the atmospheric refractive index rather than being due to wind "blowing" the light around. In that sense, it is more or less indistiguishable from the heat haze that you see on hot tarmac on a summer's day.
 
BlackRiverFalls said:
You can get Chernenkov radiation in an atmosphere, I found this account on the Insultingly Stupid Movie Physics page:
Click to expand...
A few times I've come across claims that an astronaut has been able to see Cerenkov radiation as a high energy charged particle sped through the aqueous humor of his eyeball. (He had his eyes close at the time.) Does anyone know if this is an urban legend or not? :)
 
according to nasa this is so. The astronaut sees a flash. Also reported in the article about a new effect occuring during the recent aurora caused by the solar storm, in which they said that the unusual flash seen was not one caused in the astronaut's eyeball by hi energy cosmic rays, but was entirely different.
 
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