What do we actually mean by parallel rays when talking about reflection from a concave mirror?

Question

If we place a point object at the focus of a concave mirror, we say that the reflected rays would be parallel and would meet at infinity to form an image.

What does this mean?
I tried to trace the reflected rays using the rules of image formation using a concave mirror and I got two rays almost parallel (negligible convergence). This could have been (and probably was) a construction error since achieving precision by hand is quite a difficult task to perform.

So, when we say that the reflected rays are parallel, do we mean that they are almost parallel and would meet at a lot of distance from the mirror? Is that also why we say that they will meet at infinity?

Thanks!

Does this answer your question? What does it mean to say image is formed at infinity? — , May 05 '20 at 11:09
It does answer a major part of it but I need some confirmation. So, this means that the light rays are exactly parallel and no image is formed, right? — Rajdeep Sindhu, May 05 '20 at 11:14
"So, also the image is never really formed, we still say that "the image is formed at infinity" just to maintain the symmetry of the problem. However, since this can never be measured, the wording is only a convention, not an experimental fact." — , May 05 '20 at 11:15
Thanks! And, what about when the object is placed at infinity? I know that $X_i$ and $X_o$ are interchangeable. But how would one explain it visually? — Rajdeep Sindhu, May 05 '20 at 11:21

score 1 · Accepted Answer · answered May 05 '20 at 11:57

Technically, when we say that the light rays meet at infinity, it basically means that they never meet at all. This means that the light rays will never converge, and hence never form a real image.

But the reality is a bit more complicated, and something that is not taught in school. Light behaves weirdly, both like a particle and a wave. Like a wave, it HAS to dissipate when it goes through a material medium (which we ignore in school for the sake of simplicity) (it also deviates in a vacuum, due to quantum effects). This dissipation means that at least some of the "parts" of the quanta that create a beam shall interact at a very large distance, but as that distance depends on a large variety of unmeasured factors, the last point that can be said that they will converge, is at infinite distance from the mirror!

Hence, we say that the reflected rays would be parallel and would meet at infinity to form an image.

Thanks! Pretty well explaied... – Rajdeep Sindhu May 05 '20 at 12:13 — Rajdeep Sindhu, May 05 '20 at 12:13

What do we actually mean by parallel rays when talking about reflection from a concave mirror?

1 Answers1