# How does porous anti-reflection coating work?

We know ideally that if we stick a coating of thickness $d = \frac{\lambda}{4}$ with refractive index $n_{coat} = \sqrt{n_{air}n_{glass}} \approx 1.2$ there would be no reflected wave.

But it's hard to manufacture these coatings witn $n < 1.4$. Instead, porous anti-reflection coatings are used. How do they ensure low reflection?