# Interference of 2 slit experiment when glass is put infront of 1 slit (+0.5 lambda optical path)

The question and answer is given in the image below:

I don't really understand how to work this out myself. How do I relate $+\frac{1}{2} \lambda$ to (phase difference) $\Delta\Phi = \pi$

UPDATE

I think I got it now ... isit because

\begin{align} \Delta r &= d \cdot \sin \theta \\ \frac{1}{2} \lambda &= d \cdot \sin \theta \end{align}

Since its $\frac{1}{2}$ a "half lambda" so its destructive interference?

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