Really good with physics?

Question

Ok, I need a person who's really good at physics. If you don't know please don't answer. Ok, for a concave lens there are three light rays that can pass through.
1) Any ray passing through optical centre continues without refraction
2) Any ray travelling parallel to PA (principle axis) is reflected "in line" with the PF (principal focus or focal point)
3) Any ray travelling towards the SF (secondary focal point) refracts parallel to the PA

My question is why doesn't the light ray ever go through or pass the PF?...only the extended light ray does.

Answer 1

I'm not sure of your teminology, so see if you can follow mine.
Consider a double-converging lens.
Going from left to right, there is a left hand focus, a lens, and a right hand focus. A ray parallel to the lens axis coming from the left and passing through the lens will be refracted so as to pass through the right focus, and a ray parallel to the axis coming from the right and passing through the lens will be refracted so as to pass through the left focus.

The fact that the laws of light propagation work in reverse implies that any rays passing through either focus and then passing through the lens will end up refracted parallel to the axis.

You can use these rules to get two rays from the tip of an "object" (it is usually represented as an arrow) and find where they are made to converge. This is the "image" of the point from which they were emitted. The method even works when there is no way to get a ray to go through the lens and through the focal point--ie when the object is between the focal point and the lens. The trick is to imagine the ray from the focal point through the object point--it works because the rules only depend on the angle at which the light ray hits the lens.

These ideas can be generalized to diverging (usually concave shaped) lenses. In this case the appropriate focal point is on the same side of the lens as the object.

Answer 2

1) no. Only if this ray is parallel to the PA. BUT, you can usually approximate that all rays passing through the optical center continue without refraction (you then asume a perfect lens and thin enough not to care : therefore this approximation is only valid when the incident angle is small enough)

2) only if working with a perfect lens. It is then the definition of a PF (and not "reflected", but "refracted")

3) only if working with a perfect lens. It is then the definition of a SF

4) in the case of a concave lens, the PF sould be the focal point on the side of the incoming light (sorry, I'm french, so I have to translate for myself). And if that's true, you can put any incoming ray you want (even ones that go through the PF).
However, the outcoming rays will necessarily come out the other side (assuming no reflection), so those ones canot go through the PF.
4 bis) example : a light ray placed exactly on the PA will go through both PF and SF since PF and SF are on the PA in the case of perfect lenses.

by the way, no need to be "good with physics" to know that. This is high school level physics, in france.

Answer 3

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html