when light enters the medium of a glass prism from air, the light slows down / speeds up. the speed of each wavelength changes by a different amount. these differences of speed cause different wavelengths of light to refract/ reflect by different amounts when the light hits a surface at an angle, and that's why we see separate colors of light. the greater the change in speed between two media, the greater / lesser the refraction of a light wave. so light moving from air to glass will refract more / less than light moving from water to glass