We cannot attain 100% monochromatic light because of heisenberg uncertainity principle.even a laser source doesn't have one wavelength although the bandwidth may be small instead.

can someone plz explain me why does it happen??: doh:

A single frequency is a sine wave of infinite extent. You can't create this — you have a wavetrain of finite extent, which necessarily includes a range of frequencies, even if that range is small.

 

In terms of the HUP, any transition has a finite lifetime, meaning that it has an energy width, from [math]\Delta E \Delta t > \hbar/2[/math] This is why clock people and metrologists (i.e. the people I hang out with at conferences) like long-lived transitions, because they are narrow and allow precise measurements.

isn`t a sine wave made up of lots of Square waves at different frequencies anyway?

 

I seem to remember hearing that Decades ago when I was doing my Electronics training, so I could have forgotten something pertinent.

I suppose thats true...but most people interpret it as the other way around. A square wave is composed of many sine waves.

The distinction is important because sine waves are eigenvectors or linear time invariant systems (applicable to ALOT of things) but square waves are not.

 

As to the OPs question: Swansont's answer is correct.

isn`t a sine wave made up of lots of Square waves at different frequencies anyway?

 

It's usually the other way around- A square wave (or any arbitrary waveform) can be decomposed into sine waves at integer multiples of the fundamental.

the other way around, that could be it then, I told you it was a Long time ago

 

Thanks!

You may parse this several ways. Any "steady frequency" wave packet of finite extent decomposes upon Fourier analysis into a spread, but, so what? What are the questions of physical analysis? On what time frame are we visualizing a system?