I need help in this question about electricity !?

Light from an object O enters a gl slab at B and emerges at C as shown in this diagram : http://yfrog.com/1sframerp . To an observer looking at the emerging ray, the object O appears to be at I. Since AB=OI and the rays are close to the axis , calculate the apparent displacement OI of the object , if the refractive index of gl is 1.51 and the thickness t of the gl slab is 2.3 cm.