Answer
1.2% slower
Work Step by Step
Relate the speed of light in silicate flint glass to c by using the index of refraction, v=c/n.
Calculate the percent difference by which the blue light (“b”) is slower than red light (“r”). Your answer may vary if you read the graph (Figure 24-14) differently.
$$\frac{v_b-v_r}{v_r}=\frac{c/n_b-c/n_r}{c/n_r}$$
$$\frac{ n_r- n_b}{ n_b}=\frac{ 1.62- 1.64}{1.64}=-1.2\%$$
Blue light travels about 1.2 percent slower in silicate flint glass than red light does.