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I'm looking into a concave reflector while moving an object away from reflector surface towards its focal point. I understand that when the object is at focal point, no virtual image is formed since all reflected rays are parallel. But what I saw is that the virtual image becomes very blurry way before it reaches focal point. Why is that?

If it matters, the reflector is made out of plastic with semi transparent reflective coating.

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There are a number of things that could happen:

  1. You see higher and higher effective magnification, and not always there are enough tiny features to see on the object
  2. Your mirror is likely spherical, not parabolic. That means it will suffer from especially significant aberrations which you might observe as "blurriness".
  3. You are short-sighted and cannot see clearly objects which appear far away (nearly parallel rays = objects many meters "behind the mirror").
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  • $\begingroup$ Yes, I forgot to mention that the mirror is spherical. $\endgroup$ – mr49 Nov 19 '16 at 4:13
  • $\begingroup$ Hmmm... did not know rays were parallel at the focal point. Seems to be impossible to me, but I am not an expert in the field. $\endgroup$ – Lambda Nov 19 '16 at 4:35

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