<math>A^2 _{12} / A^2 = C _1 + C _2 + 2 C _1 C _2 \cos ( d _2 - d _1 ) </math>
<math>A^2 _{12} / A^2 = C _1 + C _2 + 2 C _1 C _2 \cos ( d _2 - d _1 ) </math>
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Because the wave reflecting off the back of the diamond travels through the diamond twice, the term <math> d _2 - d _1 </math> is twice the thickness of the diamond, in radians. Because this measurement is in unhelpful units, we can multiply it by the wavelength <math> \gamma </math> and divide by <math> 2 \pi </math> for the thickness <math> \tau </math> in meters.
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Because the wave reflecting off the back of the diamond travels through the diamond twice, the term <math> d _2 - d _1 </math> is twice the thickness of the diamond, in radians. Because this measurement is in unhelpful units, we can multiply it by the wavelength <math> \lambda </math> and divide by <math> 2 \pi </math> for the thickness <math> \tau </math> in meters.
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<math>( d _2 - d _1 ) \gamma / 2 \pi = \tau </math>
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<math>\frac{( d _2 - d _1 ) \lambda}{2 \pi} = \tau </math>
Therefore, we can rewrite the earlier equation in terms of <math> \tau </math>:
Therefore, we can rewrite the earlier equation in terms of <math> \tau </math>:
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<math>A^2 _{12} / A^2 = C _1 + C _2 + 2 C _1 C _2 \cos ( 4 \pi \tau / \gamma ) </math>
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<math>A^2 _{12} / A^2 = C _1 + C _2 + 2 C _1 C _2 \cos ( \frac{4 \pi \tau}{\lambda} ) </math>