[subhan2010]

Data transmitted along glass fiber cables is in the form of pulses of monochoromatic red light each of duration 2.5 ns. Which can be the best estimate of the no. of wavelenghts in each pulse..

[ebaines]

data transmitted along glass fiber cables is in the form of pulses of monochoromatic red light each of duration 2.5 ns. which can be the best estimate of the no. of wavelenghts in each pulse..?

I assume you have access to find either the frequency of red light in cycles per second (Hz). You can the pulse time of 2.5 ns by the frequency to determine how many cycles of the wave pass by in 2.5 ns.

[subhan2010]

can I use v=f*wavelength... where f=1/T. but how will I determine the velocity. (if use this formula)

[ebaines]

Yes, you can use that. I assume you have a value for wavelength and want to determine frequency, correct? As for what velocity to use: light travels at the speed of light 'c', which is about 3 x 10^8 m/s.

[subhan2010]

I am determining wavelength.. since freq. can be obtained from time.. bt I can understand the meaning of "no. of wavelengths in EACH PULSE"

[ebaines]

Each pulse of laser light running down a fiber cable has a length to it. The length of the pulse depends on time duration that the laser was firing. For example if you turn on a laser for one second then turn it off that pulse of light will be 3x10^8 meters long. If you turn the laser on for a brief pulse, say one millionth or 10^(-6) seconds, the length of the light pulse is 3 x 10^8m/s x 10^(-6) s = 300 m. So to find the length of each pulse for your problem it's the speed of light times 2.5ns. Once you have the length of the pulse divide by the wavelength of light to determine how many waves of light are contained in one pulse.

[subhan2010]

I didn't get you.. you are saying that
length of pulse= v * T
and then divide by wavelength..
but also,
v * T= wavelength.. so how can I divide these two equal quantities.can u show the working of this whole numerical please.. (the answer is 10^6)

[ebaines]

The length of the pulse L is c times T, where T is the length of the pulse in time:

L = 3 x 10^8 m/s x 2.5 x 10^(-9)s = 0.75 m

So each pulse is a burst of light that is 0.75 meters long.

The wavelength of red light is about 0.75 m. So the number of wavelengths that fit into a pulse of light 0.75 m long is 0.75 / (7.5 x 10(-7)m) = 10^6.

[subhan2010]

thanks.. where the wavelength of red light should be known or you have calculated it.. 7.5x10^-7m

[ebaines]

thanks...!where the wavelength of red light should be known or you have calculated it...? 7.5x10^-7m

I looked it up using Google. I would guess that your text provides either the wavelength of red light or its frequency . If you know it's frequency then its wavelength can be calculated from .

[subhan2010]

Thanks again.. :D