A lens of glass is coated with a thin layer of a clear material of refractive index 1.25, which is…

Step 1. The wavelength in the coating. $\dfrac{\lambda}{n} = \dfrac{540\ \text{nm}}{1.25} = 432\ \text{nm}$.

Step 2. The least thickness. The two reflected waves cancel when $2nt = \dfrac{\lambda}{2}$. We want $t$, so $t = \dfrac{\lambda}{4n} = \dfrac{540\ \text{nm}}{4 \times 1.25} = 108\ \text{nm}$. This is a quarter of the wavelength in the coating.

Step 3. The next thickness. With $2nt = \dfrac{3\lambda}{2}$, $t = \dfrac{3\lambda}{4n} = 324\ \text{nm}$.