After light wave $1$ reflects off interface a and light wave $2$ reflects off interface $b$ they will meet up again at the surface of the thin film of oil.

When they meet up$,$ light $1$ has not traveled any distance but it has experienced a phase change. What is the effective path $l_1$ for lighti?

$2d+\frac{}{2}$

$0$

$\frac{}{2}$

$2$d