$(5$ pts$)5.$ A thin sheet of plastic with thickness $t$ and refractive index $n=2\mathrm{.}00$ is on the surface of a flat block of glass that has $n=1\mathrm{.}60.$ Light with wavelength $1200$ nm in air is at normal incidence to the plastic sheet. What is the smallest nonzero thickness $t$ of the plastic sheet for which there is constructive interference between light reflected from the top and bottom surfaces of the plastic?

$($a$)100$ nm

$($b$)150$ nm

$($c$)300$ nm

$($d$)600$ nm

$($e$)$ none of the above answers

$+n=2$

$n=1\mathrm{.}6$

$=1200$

odd phase changes

$zt=\frac{}{n}$