pick oneA) red light travels faster than blue light in glassB) Blue light travels faster than red light in glassC) Red light travels faster than blue light in airD) blue light travels faster than red light in air.
A) red light travels faster than blue light in glass The speeds through air, are so insignificantly distinct from the speeds through empty space. And the speeds through empty space are guaranteed to be identical for all frequencies. That means, it is the more optically dense material that is responsible for the frequency dispersive effects. It is very rare that you could have dispersion due to the less optically dense material's refractive index differences across the spectrum. Consider water and glass, for example. Water's refractive index, is about 1.331 (red) to 1.345 (violet). In general, glass's refractive index is about 1.5. Glass is the more optically dense than water. And you will find that even the most color-neutral glass variants that a glazier can make, differ in refractive index MORE among the colors, than water. Crown glass, is a common color neutral glass. Even so, its refractive indices range from 1.515 (red) to 1.533 (violet). This means that the water-to-glass refractive indices are 1.138 (red) to 1.139 (violet). Any effect that the water had, on reducing relative refractive index, all it did was neutralize the effect for the crown glass. If there was any dispersion due to the water rather than due to the glass, we would see red light bending more than violet light. This is why it was such a challenge, to get the first achromatic telescopes, that wouldn't disperse by color. Any lens-based telescope, would inevitably blur the colors of the subject. Until Newton came up with the idea, to FORGET using lenses as the active component, and to use mirrors instead.
