I have a question. It seems to me, when light enters glass from the air, since it is a semi circular block and the light passes through the center, so the light won't bend when entering glass. The light does bend when it hit the glass air interface again and enter air from glass. In this case, from the wording of the question, the incidence angle is equal to critical angle and refraction angle is 90 degrees, because when the spot reaches point B, it disappears. Now increase frequency, will increase both refractive index for glass and air, but the refractive index increases more in the glass, so according to the calculation, if the incidence angle won't change, the refraction angle will be greater than 90 degrees, so the point will disappear. But this contradict the question asking whether AB increases or decreases. Help me understand