that is also quite easy .. let angle of minimum deviation be d
well we know the relations
A=r1+r2 --- - -(i)
d=i+e -A
further from (i) we get
d(r1)/di = -d(r2)/di
and we know
nsin(r1)=sin(i) -- -(ii)
nsin(r2)=sin(e) -- - (iii)
=>
ncos(r1)dr(1)/d(i)=cosi
ncos(r2)d(r2)/d(i)=cos(e)(de/di)
dividing both equations
-cos(r1)/cos(r2)=(cosi/cose)(di/de)
further from (ii) and (iii)
cos(r1) = (1-(sin(i)/n)2)1/2
and
cos(r2) = (1-(sin(e)/n)2)1/2
further for d to be extremum
d(d)/di=0
thus
d(i+e-A)/di=0
thus
1+de/di=0
but we have already obtained de/di
solving this equation we get
(n2 - sin2i)/cos2i = (n2-sin2e)/cos2e
=>
(n2-sin2i-cos2i)/cos2i = (n2-sin2e-cos2e)/cos2e
thus we get two possibilities
n=1 or i=e
but n=1 is excluded( as n we cant manipulate)
and thus we get i=e for extremum
and we can see that the value of d obtained in this case is less than the value obtained when ray falls perpendicularly
thus IT HAS TO BE MINIMUM
please reply sir ..