geometric-opticslensesoptics
What will be the path of ray passing through the first focal length of a biconcave thin lens kept in air?
I know that a ray passing through focus or appearing to pass through focus of a thin lens becomes parallel to the principal axis. But in this case I cannot see how?
Assuming mirror to be spherical section. C is the center of sphere.
See, Using trigonometry. $$x=d \times \sin(2\theta)$$ $$x=R\times\sin\theta$$
Eliminate $\theta$ and get $d$ : distance from Center of curvature as a function of $x$.
Verify for small theta where $\sin\theta\approx\theta$
If you just want to see that which side ray bents then see. $$d=\dfrac{R\times \sec\theta}2$$ which shows that $d\ge R/2$ . So, ray bends towards the pole as looses it paraxial character.
If the object is at infinity, lies across the axis and some rays from it are coming in at a non zero angle, then the object is infinite. A finite image of the object is then highly diminished.
Best Answer
The ray passing from the focus, after the lens overlaps with the one that starts from the tip of the virtual image, see the image. (Basically that ray is not a "special" one).