# Why does light bend after travelling half of the lens?

As light enters a denser medium from a rarer medium, it bends towards the normal. Why does the light bend after passing through point F & not E?

• Similar earlier question on Physics: Why doesn't a light ray bend again when emerging from a lens? Commented Nov 25, 2021 at 19:46
• it doesn't, it's a simplification Commented Nov 25, 2021 at 22:18
• Refraction depends on the index of refraction. While that may be affected by density, density itself does not cause refraction (unless you mean optical density, in which case saying just "density" is misleading). Commented Nov 26, 2021 at 7:08
• This is a 'thin lens' approximation. As the lens is 'thin', it doesn't really matter whether the refraction takes place at the two faces of the lens (which is what actually happens) (see NMech's diagram), or the centre of the lens (which is a close enough simplification). Optics is hard enough for most students to grasp, so it's made easier for them by just having one deflection point, to make diagrams easier to draw. Commented Nov 26, 2021 at 20:45

It changes on both E and the exit point of the beam (see image below).

source

My understanding is that refraction occurs at any boundary that there is a change of the wave propagation velocity.

The image in the original post is a simplification which is used either as an introductory image or to describe other aspects where what happens inside the lens is not important.

• Should be E and the exit point (unlabelled), not E and F Commented Nov 25, 2021 at 19:50
• Yes you are right. My eyesight is not what it used to be. I'll correct it. Commented Nov 25, 2021 at 20:28
• More to the point, OP's diagram is commonly used when introducing and illustrating the assumptions of the thin lens approximation.
– J...
Commented Nov 25, 2021 at 21:54
• Your image is also a simplification... Optics is a weird rabbithole of ever complicating simplifications, all of which refuses to make sense untill you start reading some QM, at which point the world you realize that the universe refuses to make sense, in general. Commented Nov 26, 2021 at 9:15
• @StianYttervik I agree. I can't argue with what you are saying. Commented Nov 26, 2021 at 9:20

You are right, rays refract at both the entry and the exit points. That drawing is just a simplified diagram, frequently used in textbooks about geometrical optics. It helps to illustrate concepts like parallel rays, focal point, object distance, image distance etc. where we do not really care what happens inside the lens.