# How does light speed in a medium affect refraction?

Light rays hit a convex lens and converge at a focal point after passing through as shown by the diagram below:

It is obvious by looking here that the light rays travel different distances to the focal point but they arrive at the same time. Photons travelling along the Principal axis travel in a straight line and spend the longest time in the medium. I read that the medium (of glass in this case) slows down light due to weak interaction. An airplane slowing down would lose altitude like the light changing course in the top half of the lens. I just don't understand how the direction of the light rays gets changed in an upward direction for the bottom half of the lens.

To be very blunt lets say 5 individual photons travel along each of the 5 axes shown hitting the lens at the same time ...

A) Do they arrive at the focal point at the same time?

B) How does the slowing down in the medium cause the photons to change direction both downwards and upwards yet allows the photon on the principal axis to continue unaffected?

P.S. I can see the light rays are refracted but can't seem to link this to speed

In advance, thank you for explaining this

• Do you know Snell's law? en.wikipedia.org/wiki/Snell's_law
– user234190
Commented Jan 13, 2020 at 22:31
• @user47014 - thank you for that. I can work on it from there. It seems these kinds of diagrams are oversimplified as the exit angle from the medium should also be represented. Commented Jan 14, 2020 at 10:14