# When parallel light Ray's ( not parallel to the principal axis) pass through a convex lens, where does it converge on the focal plane?

I know that the Ray's will converge at the focal plane. But how do we calculate exactly where it will strike? Is there any formula for the same?

They converge at a point on the focal plane such that the angle between the reflected ray and principle axis is the same as the angle between the incident ray and principle axis.

Let the height of the point of convergence from the principle axis be $$h$$, focal length be $$f$$ and the angle between the incident ray and principle axis be $$\alpha$$, then we get

$$\frac h f = \tan \alpha$$

• Won't you justify why this is true?
– user258881
May 5, 2020 at 10:55
• The point of intersection of any reflected ray and the focal plane gives the point of convergence (As the Rays are paraxial we get a point image ) .Also , The ray passing through the optical center of the lens goes undeviated ; thus considering the point of intersection of this ray and the focal plane gives us the above condition
– user260622
May 5, 2020 at 11:11
• I was talking about justifying that the angle would be the same.
– user258881
May 5, 2020 at 11:12
• Yeah , sorry. Edited my comment
– user260622
May 5, 2020 at 11:14