Silicon has a refractive index of about 4.0. So, light travels upto 4 times slower than in vaccum.
Also, the delays are more than just the distance within chips since sensing and detection delays are typically larger.
Currently, the main advantage of optical interconnects is that the capacitive charging and discharging delays within the chip transistors and interconnects comprise a major portion of the total delay. This problem is exacerbated with smaller geometries such as 32nm and 28nm.
Optical interconnects do not suffer from such problems.