Thin-Film Interference Practice Test

A thin film with index greater than air lies on a substrate with the same index as air. Light reflects from the top and bottom film surfaces and recombines in air. For normal incidence, which thickness minimizes reflected light at wavelength $\lambda$?

A thin film of thickness $t$ has refractive index greater than air and is in contact with air on both sides. Light reflects from the top and bottom surfaces and recombines in air. Which thickness condition produces constructive interference in reflected light (normal incidence)?