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

enter image description here

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

  • 2
    $\begingroup$ Do you know Snell's law? en.wikipedia.org/wiki/Snell's_law $\endgroup$
    – user234190
    Commented Jan 13, 2020 at 22:31
  • $\begingroup$ @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. $\endgroup$
    – Wookie
    Commented Jan 14, 2020 at 10:14

1 Answer 1


Ray optics is a simplification tool useful for making calculations but not very useful for understanding light behavior. If you want to understand why light bends when it changes medium, you need to consider light as a wave, not as a particle. The Huygens–Fresnel principle will explain the rest.

  • $\begingroup$ Thank you for your answer. It seems I am wrong to conclude that light slows in a medium. In a medium that is non-vacuous the light must follow a path that is not straight to maintain c $\endgroup$
    – Wookie
    Commented Jan 16, 2020 at 10:21
  • 1
    $\begingroup$ @Wookie light goes straight in all homogenous media. What is slowed is the phase velocity not the speed of light:"The ratio between the speed of light c and the phase velocity vp is known as the refractive index" source en.wikipedia.org/wiki/… $\endgroup$ Commented Jan 17, 2020 at 12:14
  • $\begingroup$ Thanks, and for you ... math.rug.nl/~broer/pdf/ws-ijbc.pdf $\endgroup$
    – Wookie
    Commented Jan 17, 2020 at 21:56
  • $\begingroup$ @Wookie why did you link this paper? how is it related to my answer ? $\endgroup$ Commented Jan 20, 2020 at 1:06
  • 1
    $\begingroup$ @Wookie the curve results from the fermat's principle of "least time" (time =phase speed x distance). However that speed isnt c, it's the phase velocity (phase velocity depends on refractive index). $\endgroup$ Commented Jan 23, 2020 at 15:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.