9

1

enter image description herepg
In this picture the chief ray is not bending which is going through the optical axis. It passes straight while other rays from the same object are refracted because they change the medium.

Martin
  • 15,550
Anusha
  • 101
  • 4
    The chief ray goes through the center of the lens. Refraction effects come from the curvature of the lens. At the center there is no curvature and so the ray does not bend. – Reid Erdwien Mar 20 '15 at 07:07
  • 2
    The other rays bend towards the centre (or away from the centre in the concave case). But the chief ray is already at the centre so there's no "towards" to bend to. – IanF1 Mar 20 '15 at 07:28
  • 7
    @RdErdwien At the center there is no curvature and so the ray does not bend. - This seems incorrect. There is a non 0 angle of the chief beam and the tangent to the lens surface (in given example). The beam does bend when entering a different transmission medium. (this comment is unrelated to the question itself though, don't consider it an answer) – user Mar 20 '15 at 07:42

1 Answers1

30

This picture shows what happens to a ray passing through the center of the lens:

Light ray

The incoming ray hits the air-glass surface of the lens at an angle $\theta$ to the normal and is refracted. It then passes through the lens and hits the glass-air interface at the other side where it is refracted again. Because the lens is symmetrical about the horizontal line the angle it emerges from the lens, $\theta$, is equal to the angle it hits the lens. So the direction of the ray doesn't change, but it is displaced slightly. Strictly speaking it doesn't go straight through but we usually ignore the small displacement.

John Rennie
  • 355,118
  • 16
    Good answer. Just to clarify; the standard optics term for ignoring the thickness of the lens is the thin lens limit. – Chris Mueller Mar 20 '15 at 11:49