Inside a substance such as glass or water, light travels more slowly than it does in a vacuum. If c denotes the speed of light in a vacuum and v denotes its speed through some other substance, then $v=\frac{c}{n}$

Where n is a constant called the index of refraction.

To almost exact approximation, a substance’s index of refraction does not depend the wavelength of light. For instance, when red and blue light waves enter water, they both slow down by about the same amount. More precise measurements, however, reveal that n varies with wavelength. Table 1 presents some indices of refraction of Cutson glass, for different wavelengths of visible light. A nanometer (nm) is $10^{–9}$ meters. In a vacuum, light travesl at $c = 3.0 × 10^8 m/s$. Indices of refraction of Cutson glass (Table 1)

Inside Cutson glass

Orange light travels faster than yellow light

The velocity of light is inversely proportional to the index of refraction $v=\frac{c}{n}$, So, the lower the n, the higher the v. According to table, Cutson glass has a slightly lower n for orange light than it has for yellow light. Therefore, inside Cutson glass, orange light travels slightly faster.