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The angle of the ray coming from very far away is so small that it is treated as zero.  

Let us say the height of the object is  h.   The distance of the object from lens is d.   Then the angle that rays from the object make at the lens with the horizontal (or the axis of the lens) is  less than or equal to   tan⁻¹ h/d.    If  d >> h, the angle is nearly zero.

Also, the size of lens is so small compared to the distances involved.

So the rays are treated as parallel to the axis of the lens.

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