A glass has a refractive index of 1.509 for red light and 1.524 for violet light.

Step 1. Red. $v = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.509} = 1.988 \times 10^{8}\ \text{m/s}$.

Step 2. Violet. $v = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.524} = 1.969 \times 10^{8}\ \text{m/s}$.

Step 3. The difference. $1.988 \times 10^{8}\ \text{m/s} - 1.969 \times 10^{8}\ \text{m/s} = 1.96 \times 10^{6}\ \text{m/s}$, about $1\%$.