# Homework Help: Solar cell interference filter

1. Apr 18, 2015

### Wminus

1. The problem statement, all variables and given/known data
See attached image, assignment a)
2. Relevant equations

3. The attempt at a solution
Using ordinary trigonometry, I got simply $$\Delta = sin(\theta_i) 2 d tan(\theta_t) n_{air} - 2n_{oxide} d/cos(\theta_t)$$. However, even if I apply Snell's law, it's not the answer I'm supposed to get according to the assignment. So what's happening?

The two lower triangles with the angle $\theta_t$ are identical, right?

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• ###### Skjermbilde 2015-04-18 kl. 16.00.22.png
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2. Apr 18, 2015