If you need to calculate the effective refractive index of composite, you can use Lorenz-Lorentz equation.

But be aware that a PhC cannot typically be treted as an effective index medium, instead the photonic bands have to be calculated, which depend on the refractive index of both high and low index materials.

Hi,@ Vadapalli Durga Rama Pavan

you can use the effective index mode solver in this website to calculate the effective refractive index .

https://www.computational-photonics.eu/eims.html

you can also find explanation here:

https://www.computational-photonics.eu/eimsinout.html#LIMITS

Good luck