Parallax
In order that you can locate an image behind a plane flat mirror you need to understand the word parallax.
Definition of parallax = The effect by which the positions of an object seem to change when it is looked at from different positions.
The key words here are "seems to change position".
Here's a simple parallax experiment that explains how it works.
Hold up the index fingers of both hands and put one finger 100mm directly behind the other as follows:
If you keep your fingers still and move your head from side to side you will notice how your fingers appear to move.
Below are the fingers lined up in the sight line of your eye.
Move your head to the left 50mm.
Now move your head to the right 50mm from the centre. See below:
The above phenomenon is called PARALLAX.
This is the apparent relative motion of two objects caused by the motion of the observer.
Now let's try again, but with the fingers further apart at 300mm. Again, remember not to move your fingers, only your head.
Below are the fingers lined up in the sightline of your eye:
Again, move your head to the left only 50mm.
Notice how this time the fingers appear to have moved much further.
Now move your head to the right 50mm from the centre. See below:
Again, notice how the fingers appeared to move much further.
The above is a visual experiment to show parallax. This can be observed when you also drive down a street and a tree in front of a house appears to move across the front of the house as you drive by.
