Snell's law gives the relationship between the angle of incidence and the angle of refraction:
[tex]n_i \sin \theta_i = n_r \sin \theta_r[/tex]
where
[tex]n_i[/tex] is the refractive index of the first medium
[tex]n_r[/tex] is the refractive index of the second medium
[tex]\theta_i[/tex] is the angle of incidence
[tex]\theta_r[/tex] is the angle of refraction
In our problem,
[tex]n_i =1.61[/tex]
[tex]n_r=1.36[/tex]
[tex]\theta_r =25.6^{\circ}[/tex]
So if we re-arrange Snell's law and we use these data, we can find the angle of incidence:
[tex]\sin \theta_i = \frac{n_r}{n_i} \sin \theta_r = \frac{1.36}{1.61} \sin (25.6^{\circ}) = 0.365 [/tex]
which gives
[tex]\theta_i = \arcsin(0.365)=21.4^{\circ}[/tex]
