Physics question

[silverpig]

Some more info

Physical waves of a given type in a given medium generally exhibit a characteristic group velocity as well as a characteristic phase velocity. This is because within a given medium there is a fixed relationship between the wave number and the frequency of waves. For example, in a transparent optical medium the refractive index n is defined as the ratio c/vp where c is the speed of light in vacuum and vp is the phase velocity of light in that medium. Now, since vp = w/k, we have w = kc/n. Bearing in mind that the refractive index is typically a function of the frequency (resulting in the "dispersion" of colors seen in prisms, rainbows, etc), we can take the derivative of w as follows...

In a medium whose refractive index is constant, independent of frequency (such as the vacuum), we have dn/dk = 0 and therefore the group velocity equals the phase velocity. On the other hand, in most commonly observable transparent media (such as air, water, glass, etc.) at optical frequencies have a refractive indices that increase slightly as a function of wave number and (therefore) frequency. This is why the high frequency (blue) components of a beam of white light are deflected more than the low frequency (red) components as they pass through a glass prism. It follows that the group velocity of light in such media (called dispersive) is less than the phase velocity.

Cerenkov radiation

 

[MikeMike]

light changes speed depending upon the medium it is located in.

thus you get refraction, and reflection.

do thi. put a square piece of plexiglass on a white piece of paper. draw a diagonal line on one side of the block, then looking through the block, draw the next one as straight as possible as if they were connected.

remove the block. right there you have changed the speed of light, thus it refracted and your lines are not "parallel" rather require a diagonal line between them to connect.

MIKE

 

[Sahakiel]

In answer to the original post :

Speed is a scalar quantity. No direction.
Velocity is a vector. Has direction.
Don't mix them up.

 

[TheNinja]

E = MC^2

 

[CanOWorms]

I'm by no means an expert in photonics, but I believe Silverpig and Heisenberg are correct, too.

 

[RossGr]

Suppose you are driving through a town with many stop lights. Between each light your speed is 20mph. The lights are timed so you must stop at each one. What is your average speed though town? How fast did you drive? Are these equal numbers? No it takes much longer to drive the same distance if you have to stop every 2 blocks.

Light passing through a medium is stopped by each atom it encounters, after a short pause, a new photon is re-emitted to continue the journey. Between atoms it travels at c. So even in a medium the speed of light is c, even though on a larger scale it appears to be slower.

 

[HomeBrewerDude]

Originally posted by: Sahakiel
In answer to the original post :

Speed is a scalar quantity. No direction.
Velocity is a vector. Has direction.
Don't mix them up.

thank you

 

[ClueLis]

Originally posted by: Skoorb

Originally posted by: silverpig

Originally posted by: Skoorb

Originally posted by: silverpig

Originally posted by: HomeBrewerDude
i was thinking about this the other day.


Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

Acceleration changes velocity, but not necessarily speed. If you run in a circle at a constant speed, you are constantly changing your velocity.

That's fine. Now, how does light maintain a constant speed if it's being pulled by a black hole?

The black hole just curves space as does any other massive object. Light must travel along this curvature.

Picture a car that can only go 60 mph. You put it on flat land, it goes 60, you put it on a curvy road, it goes 60. Now, a black hole (or star, planet etc) would be similar to putting a huge depression in the earth, like a funnel shape. The car must travel along this curve. It still goes 60 with respect to the road it's travelling on.

Well then how can gravity affect light?

General relativity proposes that gravity bends space, so that the light is still moving in a straight line, but appears to be bending from our perspective because the space itself is not "flat". The biggest mental step here is that space is not always Euclidian (i.e. the shortest distance between two points isn't always a straight line).