Chromatic Aberration
Putting these two ideas together, we come up with the prediction that the focal length of lenses will be slightly different for different colors!
This means that a single lens will not be able to focus white light perfectly. We actually observe this, and it is called Chromatic Aberration.
To reduce this, we can employ two lenses with different materials to replace a single lens. This adds to the expense and is one reason why good cameras are so expensive.