When white light goes through a prism, it undergoes refraction and the
different colors of white light are separated. this occurs
•the light is slowed down by the prism, but the different wavelengths of light are
all slowed down a different amount by the prism.
•the light is slowed down by the prism, but all wavelengths are slowed down the
same amount.

The light is slowed down by the prism, but the different wavelengths of light are  all slowed down a different amount by the prism.

Explanation:

What happens here is that white light passes from a medium with an specific index of refraction (air) to another medium with a different index of refraction (the material of which the prism is composed).

Then, when light passes from a medium to another, its speed changes causing the deviation of the beam of light. However, we are talking about white light that is composed of different wavelenghts. So, since the frequency of light remains costant when passing from one medium to another, the different wavelengths change as a result of the change in speed.

This was first observed by Isaac Newton in the eighteenth century, when he found out that when a beam of light from the Sun, passes trhough a prism is decomposed in many different colors.

a and b sometimes have a positive value