Refractive index
n21 = n2/n1 OR. n1/n2
Kindly refer to the link given below to get your answer :
https://www.meritnation.com/ask-answer/question/is-n21-n1-n2-or-n2-n1/light-reflection-and-refraction/8818411
https://www.meritnation.com/ask-answer/question/is-n21-n1-n2-or-n2-n1/light-reflection-and-refraction/8818411