An optical device having at least two optically transparent optical parts which are bonded together with an optically transparent adhesive layer. The adhesive layer is formed from a matrix containing silicon, titanium, zirconium, aluminum or germianium, and oxygen atoms. At least part of the atoms is bonded to other atom(s) through a polyvalent hydrocarbon group and directly bonded a monovalent hydrocarbon group. The refractive index value of the adhesive layer approximates to the refractive index values of the at least two optically transparent optical parts. An adhesive composition for bonding optical parts and a method of bonding optical parts are also proposed.