What is the longest wavelength of light, in nanometers, that will undergo destructive interference when it is shone on the oil

Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.

The chemical reactions lab we did is typically called a "copper plating lab,  because the acid solution cleaned the pennies and made the copper look shinier;   the acid solution cleaned the paperclips and made them look like pennies;   &   the copper from the pennies was removed, and then added to the paperclips.