Hi Sonali,

Optical instruments are practical application of optics. In order to understand the principles behind their making and to know them better you first need to have understood the concepts of reflection, refraction, mirrors, lens, lens diagram (it is must), laws of reflection and refraction and other related concepts. Hence it is advisable to study ray optics first ao that you could understand optical instruments in a more simplified and better way.

I hope that you understood. Wish you all the best.