State what phase change occurs on reflection at the air-coating boundary and at the coating-glass boundary.

The thickness d of the coating layer is 110 nm.

Determine the wavelength for which there is no resultant reflection from the surface of the lens for light at normal incidence (θ = 0º).

180º / π;

path difference must be $\frac{\lambda }{2}$;

physical thickness must be $\frac{\lambda }{2n}$;

so, maximum wavelength $\left( {{\rm{is}}\left[ {2nt = \left[ {m + \frac{1}{2}} \right]\lambda } \right] \to \lambda  = 4{n_c}d} \right) = 528\left( {{\rm{nm}}} \right)$;

Allow any valid alternative method.

was well done in general

Was poorly understood. There were some good answers but many candidates omitted n or put n = 1 as in a diffraction grating.