[Physics] Why does red light travel faster than blue light

Question

I know that light of all frequencies travel at the same speed in vacuum.

But I wonder why their speed differ in any other medium, why does red light travel faster if it has less energy than blue light?

Answer 1

Jon Custer hinted at something, which I think is best explained via an analogy.

Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. But when the pavement is crowded, you're dodging around people and bumping into them. You're still walking at 4mph, but it takes you an hour and a half to travel the four miles. And if you're a little old lady with short little steps walking at 4mph, you're held up more than if you're a big guy with long strides walking at 4mph. Now let's look at your questions again:

But I wonder why their speeds differ in any other medium?

Because the light interacts with the material, and those interactions are wavelength dependent.

And why Red light travels faster while it has less energy than blue light?

Because the light interacts with the material, and those interactions are wavelength dependent!