Plural of "Refractive Index": Debate

[tomeatworld]

Homework Statement

So I'm starting to write a lab report but I've hit a boundary in that I have inflicting opinions on what the plural of 'refractive index' is. I have some people saying 'refractive indices' and others saying 'refractive indicies' and a few people saying 'refractive indexes'. I'm pretty sure it's not the latter but i have no idea which of the first two. Spelling really does matter in this thing.

Thanks in advance

 

[nasu]

Homework Statement

So I'm starting to write a lab report but I've hit a boundary in that I have inflicting opinions on what the plural of 'refractive index' is. I have some people saying 'refractive indices' and others saying 'refractive indicies' and a few people saying 'refractive indexes'. I'm pretty sure it's not the latter but i have no idea which of the first two. Spelling really does matter in this thing.

Thanks in advance

It's index - indices (in American and Canadian)
It may vary in different versions of English.
And by the way, it's actually "grammar".

 

[tomeatworld]

hahahaha. Oh the irony. Thanks a lot for the answer!