We are in a way saying the same thing; I need to find Io first then substitute in the formula above after finding I, the formula you said can now work.
n1sintheta1=n2sintheta2
1*sin36=n2sin24
n2=sin36/sin24
n2= 1.45
to calculate wavelength is lambda=c/f
My question is that that how do I find the wavelength without given the frequency.