According to Snell's law, when a light ray incidents on a surface separating two transparent media, the product μsinθ of a light ray is constant.
where, μ is the refractive index of the medium and \theta is the angle of incidence/refraction.
Given, angle of refraction is double the angle of incidence
→r=2i
light ray passes from a glass of refractive index μ=1.6 to air.
⟹1.6sin(i)=sin(2i)=2sin(i)cos(i)
⟹cos(i)=0.8
⟹i=cos−1(0.8)=sin−1(0.6)
Therefore, angle of incidence is sin−1(35) .