According to the law of refraction, it can be concluded that the angle between the light and the vertical direction is 3 after it enters the glass. The light passing through O is vertically incident on the AB interface and exits at point C, and no light exits between C and B; The closer to A, the greater the incident angle of light incident on AB interface, and the greater the possibility of total reflection. According to the critical angle formula, the critical angle is 45. If the critical point on AB interface is D, the light is incident on point E on AO interface, and the angle between OD and horizontal direction can be found to be 18-(12+45) = 15 in triangle ODE, so no light exits between A and D..

from this, it can be concluded that the central angle of an arc without light is 9-(3+15) = 45, which is 1/4 r.