A triangular medium has varying refracting index n=n0+ax, where x is the distance (in cm) along x-axis from origin and n0=43. A ray is incident normally on face OA at the mid-point of OA. The range of a so that light does not escape through face AB when it falls first time on the face AB (OA = 4 cm, OB = 3 cm and AB = 5 cm): (Surrounding medium is air)