1. na/nw=va/vw2. The critical angle is the angle of incidence above which total internal reflection occurs. The angle between the normal and the ray is 90 degrees.3. The formula is derived because the angle between the normal on the less dense medium on top of the ray(refracted angle) is 90 degrees and the incident angle will form the formula of n = sin i/ sin r which is equals to 1 so if the values are rearranged, critical angle = 1/n.
sin i/ sin r = na/nwsince na = 1 and sin90 = 1, we havesin i = 1/nw which is the formula for critical angle
If you work out the answer, i = 48.6 degreesJust memorise it. Use this value to check for water-air interface.