In any region of a space, the gravitational field cannot be eliminated. The speed of light in a vacuum has never been observed and cannot be observed with current technology. Till now, only the speed of light in a gravitational field has been observed. Here, it is presented that light could be dispersion in a gravitational field analogous to the dispersion of light in the Newtonian prism experiment. The relativistic mass density on the surface of a neutron star is on the level of 10¹⁷kgm^-3 while on the surface of the Earth is only 6.63*10^-7kgm^-3, the speed of light acted by the gravitational field of a neutron star is much larger than that by the Earth. Therefore, light dispersion in strong gravitational field could be generally observed from the picture of a star and it should have been observed through the spectroscopic binary system.