For a ray travelling from one medium into another, n_{1} sin θ_{1} = n_{2} sin θ_{2} _{1} refers to incident side; _{2} refers to refracting side n = Index of refraction of medium θ = Incident angle to medium
Sometimes when solving for θ_{1} or θ_{2}, it cannot be defined. For example, if θ_{1} = sin^{-1} [ ( n_{2} sin θ_{2} ) / n_{1} ] and is greater than 1. Secant of any value greater than one is impossible. In this case, it means there is total internal reflection and the ray never exits the medium. You can take the θ_{1} to be 90° in this case. |