Why image is formed at focus when object is placed at infinity?

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Solution

When the object is placed at infinity, the two rays AB and DE running parallel to the principal axis get reflected at point B and E respectively and intersect each other at the principal focus F on the principal axis. Therefore, in this case the image is formed at the principal focus which is highly diminished, real and inverted.

Since the rays cannot bend coming from a long .Distance without hitting any obstacles it will be straight and will be parallel.. so thats why we are saying the rays are almost parallel

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