Harsh Zala If by 'lesser diffraction' you mean 'laser diffraction' then I can help you.

The mean RI will be the weighted volume proportions of the RI's of the constituents (because light travels through a volume of space). This is also a consequence of the Gladstone-Dale rules. In practical terms this means that any constituent with a volume of < 10% of the total volume is unlikely to exert much effect on the final refractive index (as usually only 2 decimal places are required for the real and an order of magnitude for the imaginary). You can follow a series of 3 webinars (free registration required) that will take you through the basics with simple calculations. See:

Laser Diffraction Masterclass: Why do you Need Material Optical Properties?

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W200211LDBasics1.html

Laser Diffraction Masterclass 2: How Can Material Optical Properties be Calculated/Estimated

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W200317LDBasics2.html

Laser Diffraction Masterclass 3: Optical Properties - How Can Material Optical Properties be Measured

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W200414OpticalProp.html