While calculating Refractive Index,Why we have to use the formula Real Depth/Apparent Depth,why not Apparent Depth/Real Depth

We can demonstrate this in the event that you dont comprehend the equation. 

Assume MM is a point protest at a genuine profundity MAMA underneath the free surface of water XYXY in a tank. 

A beam of light occurrence on XYXY ordinarily along MAMA passes straight along MAA′MAA′.Another beam of light from MM episode at ∠i∠i on XYXY,along MBMB makes tracks in an opposite direction from typical and is refracted at ∠r∠r along BCBC.If we deliver BCBC we will find that it meets OAOA at LL.Therefore LL is virtual picture of MM which shows up when we see from CC.Now the clear profundity is ALAL. 

∠AMB=∠MBN′ 

∠AMB=∠MBN′ 

∠ALB=∠NBC 

∠ALB=∠NBC 

In ∆AMBAMB, 

sini=ABMB 

sin⁡i=ABMB 

In ∆IABIAB, 

sinr=ABLB 

sin⁡r=ABLB 

Presently, 

aμw=ABLB×MBAB=MBLB 

aμw=ABLB×MBAB=MBLB 

Assume that ∠i−−>0∠i−−>0 then B will close to A 

Along these lines, 

aμw=MALA=real depthapparent