To answer these questions you need a camera not just an ordinary camera an imaginary,invisible camera
{ i.e i(camera) i,as in x+iy not in iPhone (pun intended)}
Now take a picture of yourself (No,!! not a selfie!!)with that camera at a distance x and at height of your eye from ground. What do you see?
you will see a picture of 'beautiful you' in that picture with your left and right unswapped
Now place a mirror(a big one) facing you at a distance x/2 from you while you are standing still just the way you were. ( nowNow you know why I said,no no selfie)
Now again with you still standing in the same position take a picture of the mirror with the cam atexactly where your eye is (I know it's practically not possible but trust me this 'i' can do anything) what do you see in this photo now? You will see the same 'you' as beautiful as you were in the first pic but with your left and right swapped
It's because of you.It's It's because you did something when you were taking the second pic. You must be thinking 'what did I do? I barely moved'
If you remember, After you have inserted the mirror, when II've asked you take the second pic, how did you bring the cam from x to you and make it face the mirror?
You brought the cam to you with its lens still facing you and turned the camera around by 180 ° towards the mirror about the verticalvertical axis (bringing camera to you and turning need not be in the same order) that's why the image is looking flipped left-rightleft-right.
why did you turn it that way? why not the other way aroundabout horizontal? Had you turned the camera 180 degrees about the horizontal axis we would have seen a vertically inverted picturephotograph of you.
You took it for granted that the camera has to be turned that way only because it doesn't make any sense taking pics with upside down camera,It's like looking at the world with legs up and head down.Yes,nobody nobody looks at it that way but that that's what makes the difference.
What do you see now? If you look at the pic in the camera It's again 'you',beautiful beautiful as ever but youryou are upside down i.e vertically inverted(you are also flipped left right but that another story, it's because we define left and right based on our top and bottom ),you'll you'll notice it's not just you but also the whole world that is upside down.(Caution:Normal digital cameras and mobile cameras sometimes correct the orientation for you using gravity sensor and show upright image of you)
Still didn't get it? Thinking wherewhy the hell hasam I talking about this camera come from when we're taking about mirrors?
It's not a camera it's your point of view.
Getting back to the first question
AndBut why are we all assuming that the image is flipped left right and why is it so hard to imagine that I'm vertically flipped and not horizontally flipped even after being told? Well It's because when you flip your image horizontally your shape is almost the same except for your left is right. But when you flip vertically your shape is not retained. This is because our image is symmetric horizontally but not vertically.
Had we humans been vertically symmetric like in the image below (I know,some that some important organs are missing) it would have been easy for you to imagine that your are vertically flipped.