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Re: [Phys-l] Transparencies and phase retardation of wave



At 08:34 AM 4/7/2007, you wrote:
I am having problems with understanding how phase and intensity works
and would appreciate your comments on my summary and questions below
of what happens when light passes through a transparency.

With reference to Hecht p618 I understand that a transparency does
not cause amplitude modulation of a wave, just a phase modulation.
We cannot see the effect of this by viewing the phase retarded wave
on a screen. This phase retarded wave can be thought of as consisting
of the incident wave and a diffracted wave from the transparency
(called phase object by Hecht).


EM waves can be difficult to visualize as EM bundles of varied phase and
polarization. VHF radiation can be easier to deal with, for me.

If I arrange a short radio link of a signal at 200 MHz between two vertical
dipoles, with a side channel carrying the same signal by coax as a master,
what would happen if I interposed a huge chunk of wax between the antennas?
Comparing the master signal with the received signal after the change, I see
that there is a step change in the signal phase, and a step change in its
amplitude.

The wax block has evidently changed the phase of the signal, and I suppose
the transmission speed was slowed by the wax. The wax block also seems to
have absorbed some of the signal, reducing its amplitude.

Why does he just say one diffracted wave and not a whole load of plane
waves travelling in different directions representing modes? Like you would
see from a line of souces oscillating in phase.

Hecht states that there is a phase difference between the two of pi/2....

Why is it this and not another value?

I sometimes think there is a flavor of physicists who are mathematicians manque's.
You will easily see that any phase angle of signal can be provided by mixing the
appropriate proportions of two signals of the same frequency and polarization
at 90 degrees phase difference. It is one way of describing the observable effect.


and so one wave lags the other in terms of the phasor diagram. I now have
two waves of the same frequency whose E fields are perpendicular to each other.

Hmmmm...an alternating wave can have a ninety degree phase delay compared with
another, and an alternating wave can have a polarization angle (direction of e field)
at 90 degrees to another wave of the same phase. It is easy to confuse
polarization angle and phase angle.

The sum of these, which we see at the screen, produces the equation of the ellipse.
Using the phase difference given the light is circularly polarised at the screen.
So the intensity of the light is just the sum of the two individual waves squared
with no interference term.

We can produce an amplitude modulation at the screen by introducing a further
phase retardation so the two E fields cause interference pattern...
How do we this.. Hecht doesn't say...


Not quite sure what physical effect is being explained here. That means, if your
reporting is accurate, that the explanation is not working very well.

Thanks for getting through to the end and I hope I explained my sticking
points clearly.

Yours Sincerely
ALex


Brian Whatcott Altus OK Eureka!