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Here is my question: Take an object A and another object B, both of the same size.

B is rigid (static) but A is mobile . If I took A further away from observer then it goes on shrinking. Why is that the case ??😶😶

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If I understand your question correctly, you wonder why when an object is far away it seems smaller?

This can be explained with basic trigonometry.

enter image description here

In this triangle, let say your object A has a height given by the distance $h=HF$

If we call the distance between your eyes and the object $d=EF$

Then the height that you see is linked to the angle at the corner $E$. Let's call that angle $\theta$.

Then, you have $\theta=arctan(h/d)$

As d gets bigger, the angle $\theta$ will decrease and it will seem that the objects is shrinking.

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    $\begingroup$ bit.ly/22W7ep9 shows a 27 second clip from a TV show called Father Ted, explaining the same point about cows. $\endgroup$
    – user108787
    Aug 6, 2016 at 14:56

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