Find the intensity as function of y (interference between two propagating waves)

AI Thread Summary
The discussion centers on finding the intensity of interference between a spherical wave and a plane wave at a flat screen. The derived expression for intensity includes a cosine term, leading to confusion about whether the cosine should be squared. Participants agree that the intensity expression should not yield negative values, indicating a potential error in the book's solution. After reviewing the calculations, one contributor realizes they overlooked the square in their own answer, suggesting that the book's answer may indeed be incorrect. The conversation concludes with a consensus on the need for verification of the intensity formula.
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Homework Statement
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Let a spherical wave propagate from the origin, $y = ADcos(wt-2\pi r/ \lambda)/r$. Also, let a plane wave propagate parallel to the x axis, $y = Acos(wt-2\pi r/ \lambda)$. At x = D there is a flat screen perpendicular to the x axis. Find the interference at the point y on the screen as function of the interference at y=0.$$ E = (A e^{i(wt-2\pi D/ \lambda)} + AD/r e^{i(wt-2\pi r/ \lambda)} ) $$

$$ I = Q* (A^2 + (AD/r )^2 + 2AD*A/r cos{(2\pi D/ \lambda -2\pi r/ \lambda)} ) $$
Now, i pretend to use $$r \approx D$$ outside the cos, and $$r = \sqrt{ D^2 + y^2} \approx D(1+(y^2)/(2*D^2))$$ inside it.
So that $$ I = Q* (A^2 + A^2 + 2A^2 cos{(2\pi (y^2)/(2*D \lambda))} ) = 2A^2(1+cos{(2\pi (y^2)/(2*D \lambda))}) = 4A^2cos(\pi (y^2)/(2*D \lambda) = I_o cos(\pi (y^2)/(2D \lambda)) $$

THe problem is that the solutions is ##I_o cos(\pi (y^2)/(D \lambda)) ## And i have no idea where is my mistake.
 

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Herculi said:
$$2A^2(1+cos{(2\pi (y^2)/(2*D \lambda))}) = 4A^2cos(\pi (y^2)/(2*D \lambda) $$
Shouldn't the cosine function be squared on the right-hand side?

Otherwise, your work looks correct to me. In particular, I think the factor of 2 in the denominator of the argument of the cosine function is correct.
Herculi said:
THe problem is that the solutions is ##I_o cos(\pi (y^2)/(D \lambda)) ## And i have no idea where is my mistake.
Did they have a square on the cosine function? If not, it is clearly wrong since the intensity cannot ever be negative.
 
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TSny said:
Shouldn't the cosine function be squared on the right-hand side?

Otherwise, your work looks correct to me. In particular, I think the factor of 2 in the denominator of the argument of the cosine function is correct.

Did they have a square on the cosine function? If not, it is clearly wrong since the intensity cannot ever be negative.
Yes, i forgot the square in both answer. Thank you, so aparenttly the answer given by the book is wrong in fact.
 
Herculi said:
Thank you, so aparenttly the answer given by the book is wrong in fact.
It appears that way to me. Maybe someone else will confirm it.
 
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