The answer depends on a number of parameters. As already pointed out by others there are different types of optical glasses. It also depends on the wavelength because glass is dispersive (so the refractive index will be different in the red part of the spectrum (say 700 nm) and the blue part (around 400 nm). This is also known as chromatic dispersion. The refractive index also changes slightly with temperature. Another parameter is the light intensity – this is a nonlinear effect that happens at high intensities where the refractive index changes slightly by an amount n2*I (where n2 is the nonlinear index and I is the optical intensity). This is known as the Kerr effect and is responsible for some interesting effects such as self-phase modulation and self focusing.
So to get an answer to your question, at a minimum you should specify type of glass and wavelength (and assume room temperature and low light intensity).
