Dispersive Power

Snell's law of refraction states that the product $n\theta$ is the same on both sides of an interface. Here, $\theta$ is the angle between the light ray and the normal,, and $n$ the index of refraction.

Since $n_{\text{air}} < n_{\text{water}} < n_{\text{glass}}$ , it follows that $\theta_{\text{air}} > \theta_{\text{water}} > \theta_{\text{glass}}$ for comparable light rays. It follows that $\Delta\theta_{\text{water}\to\text{glass}} < \Delta\theta_{\text{air}\to\text{glass}}$ .

Or, since the transition from glass to water involves a smaller change in refractive index $n$ , it will also result in a smaller change in direction angle $\theta$ , leading to decreased dispersion.