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EnlightenedOne
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Homework Statement
I am working on a lab report for a Microwave Optics lab in a Modern Physics class. There is a part in it where I am supposed to derive [itex]P_{avg} \propto 1 + cos(δ) [/itex]. Here is the problem:
The electric field of the signal reflected off the fixed mirror can be written as [itex]E_1 =
E_0 cos(ωt)[/itex]. The electric field of the signal reflected off the adjustable mirror is then [itex]E_2 = E_0 cos(ωt+δ)[/itex], where [itex]δ = 2π(2x)/λ[/itex] is the phase difference in radians caused by moving the mirror a distance [itex]x[/itex]. The factor of [itex]2[/itex] comes from the fact that moving the mirror a distance [itex]x[/itex] changes the round-trip path length by [itex]2x[/itex]. These two signals combine at the detector, producing a net electric field that is equal to their sum, [itex]E_{tot} = E_1+E_2[/itex]. (We assume that both signals have the same polarization.) The detector measures power, which is proportional to the square of the total electric field, [itex]P \propto E_{tot}^2[/itex]. Using the trigonometric product identity [itex]cos(α)cos(β) = ½[cos(α+β)+cos(α-β)][/itex], show that the time-average power at the detector can be written as [itex]P_{avg} \propto 1 + cos(δ) [/itex].
Homework Equations
[tex]E_1 =
E_0 cos(ωt)[/tex]
[tex]E_2 = E_0 cos(ωt+δ)[/tex]
[tex]δ = 2π(2x)/λ[/tex]
[tex]E_{tot} = E_1+E_2[/tex]
[tex]P \propto E_{tot}^2[/tex]
[tex]cos(α)cos(β) = ½[cos(α+β)+cos(α-β)][/tex]
[tex]P_{avg} \propto 1 + cos(δ) [/tex]
The Attempt at a Solution
I really don't know where to start on this problem, but here is my preliminary attempt:
[tex]E_{tot} = E_1+E_2
= E_0 cos(ωt) + E_0 cos(ωt+δ)[/tex]
[tex]= E_0 [cos(ωt) + cos(ωt+δ)][/tex]
[tex]E_{tot}^2 = E_0^2 [cos(ωt) + cos(ωt+δ)]^2[/tex]
[tex]= E_0^2 [cos^2(ωt) + 2cos(ωt)cos(ωt+δ) + cos^2(ωt+δ)][/tex]
Then using [itex]cos(α)cos(β) = ½[cos(α+β)+cos(α-β)][/itex]:
[tex]= E_0^2 [cos^2(ωt) + cos^2(ωt+δ)+ cos(2ωt+δ)+ cos(-δ)][/tex]
[tex]= E_0^2 [cos^2(ωt) + cos^2(ωt+δ)+ cos(2ωt+δ)+ cos(δ)][/tex]
But, I really don't know where I am going or what to do and I need help. What I have for my attempt is just some "playing around" to see if something would come together. I especially don't know where "time-avg" power is coming in. But, overall, I just have no clue.
Can someone please help me figure out what to do?
Thank you!