Realistically, the laser will not miraculously split in two upon reaching the diamond, creating one wave that reflects back and a second that reflects off of the back of the diamond and then passes perfectly through the front. Internal reflection will occur; we must calculate how much there will be and whether or not we must compensate for it.
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The coefficient of reflectivity R can be calculated from
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<math> R = \frac{n _2 - n _1}{n _2 + n _1}^2</math>
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where <math> n _1 </math> is the index of refraction for air and <math> n _2 </math> is the index of refraction for diamond. We can look these up and calculate R: