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[Phys-l] Transparencies and phase retardation of wave



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).
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?

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. 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...

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

Yours Sincerely
ALex




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