Solution to task 506 from Chertov´s for extramural stud

Solution to task 506 from Chertov´s for extramural stud

506. A thin layer of transparent substance with a refractive index n = 1.3 is deposited on a glass plate. The plate is illuminated by a parallel beam of monochromatic light with a wavelength λ = 640 nm, incident on the plate normally. What is the minimum layer thickness dmin for the reflected beam to have the lowest brightness?