Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 01/05/2007 16:54:00

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: neilep on 01/05/2007 16:54:00

Dear Physics Mentors,

Light is well fast isn't it ?......I'm a fast runner and even I can not ooutrun it !

I can't even blink my eyes fast enough to not let any light it ...........even my son's scalextric can't keep up !!

But I've heard peeps referring to the speed of light in a vacuum  *cue hoover related light gag*

...so how fast does light travel NOT in a vacuum then ?

Is it possible to slow it down in a very dense translucent material *cue joke about wifeys jelly*

So, how fast does the light travel when I turn the light on in my study ?

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: eric l on 01/05/2007 17:24:34

Quote from: neilep on 01/05/2007 16:54:00

Is it possible to slow it down in a very dense translucent material *cue joke about wifeys jelly*

So, how fast does the light travel when I turn the light on in my study ?

It is actually this slowing down that causes diffraction, e.g. as light passes from air into glass or water.  A higher reffractive index means more slowing down.  But, if you ask me for speeds in different media, in German I would say "Ich bin überfragt".

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: neilep on 01/05/2007 17:31:56

Quote from: eric l on 01/05/2007 17:24:34

Quote from: neilep on 01/05/2007 16:54:00

Is it possible to slow it down in a very dense translucent material *cue joke about wifeys jelly*

So, how fast does the light travel when I turn the light on in my study ?

It is actually this slowing down that causes diffraction, e.g. as light passes from air into glass or water.  A higher reffractive index means more slowing down.  But, if you ask me for speeds in different media, in German I would say "Ich bin überfragt".

Hmm...(Ich bin überfragt)........ewe are over in demand ?..LOL....well....we wants ewe here !!!....we demand you presence here Mr Eric sir who we luff !!

THANK YOU for your comments and increasing my knowledge base about the diffraction of light stuff.....I did not realise that diffraction was a cause of the slowing down of light..I just assumed it was split !!

Hmmmmm...does that mean that the different constituents of light travel at different speeds ? ...oh my poor giddy aunt !!..*head ache*

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: eric l on 01/05/2007 17:47:14

It is the other way around :  rather than diffraction causing slowing down, diffraction is caused by this slowing down. 
In a medium (= not in vacuum) the different constituents (or colours) travel at different speeds because they represent different energy levels.

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: neilep on 01/05/2007 18:04:45

Quote from: eric l on 01/05/2007 17:47:14

It is the other way around :  rather than diffraction causing slowing down, diffraction is caused by this slowing down. 
In a medium (= not in vacuum) the different constituents (or colours) travel at different speeds because they represent different energy levels.

DOH !!...you're right Eric..My apologies ..I did in fact mean to say that the diffraction was a result of the slowing down.....Ahh !!...which as you then say because they represent different energy levels I can see now why this happens...THANK YOU VERY MUCH !!

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: Bored chemist on 01/05/2007 19:51:10

Diffraction is caused by the wave like nature of light as it goes past an obstruction.

Refraction is caused by it slowing down (and speeding up) as it goes from one medium to another. For example it travels about 3/4 as fast in water as in air. The extent to which it slows down is wavelength dependent, which is how prisms work to make pretty colours. (The pretty colours on CDs are due to difraction).

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: neilep on 01/05/2007 19:54:29

THANK EWE BC....what is it that speeds it up again once it's left  a  'slowing' medium ?...it must be propelled by something yes ?

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: RD on 02/05/2007 15:52:54

Quote from: Bored chemist on 01/05/2007 19:51:10

The extent to which it slows down is wavelength dependent, which is how prisms work to make pretty colours. (The pretty colours on CDs are due to difraction).

Prisms create spectra by dispersion, CDs create spectra by interference.

 

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: lightarrow on 02/05/2007 16:40:06

Quote from: neilep on 01/05/2007 19:54:29

THANK EWE BC....what is it that speeds it up again once it's left  a  'slowing' medium ?...it must be propelled by something yes ?

Photons inside the medium are not the same photons which leave it, so they are not speed up. Light cannot be accelerated, actually.

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: lightarrow on 02/05/2007 16:50:37

Quote from: neilep on 01/05/2007 16:54:00

So, how fast does the light travel when I turn the light on in my study ?

About 299,704,345 m/s
(in the void is 299,792,458 m/s).

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: Batroost on 02/05/2007 19:25:04

It is possible to slow down light very significantly...

e.g. http://physicsweb.org/articles/news/9/3/10 (http://physicsweb.org/articles/news/9/3/10)

Title: Light....it's well fast !!..not even a cheetah can beat it !!
Post by: neilep on 02/05/2007 22:48:17

Quote from: lightarrow on 02/05/2007 16:50:37

Quote from: neilep on 01/05/2007 16:54:00

So, how fast does the light travel when I turn the light on in my study ?

About 299,704,345 m/s
(in the void is 299,792,458 m/s).

So the difference is in fact negligible....THANK YOU