What if I say that the mirror doesn't flip left and right? You've heard it right the mirror doesn't do the flipping. As the above answers say the mirror shows what is right infront of it. It's you(we humans) who think it is flipping.
**Let me get this in detail**
Before we begin tell me ,
**'What makes you think that the mirror flips your left and right?'**
and
**'what image do you think the mirror flips? in the first place'**
The second question is a follow up question of the first one.
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. ( Now you know why I said, no selfie)
- Now again with you still standing in the same position take a picture of the mirror with the cam exactly 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**
Now you see that the image is a flipped left and right.
(I guess you can answer the second question now)
Now, your question is why didn't the camera flip the up and down, right?
I'm getting there.
It's because of you. 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, I'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 *vertical* axis** (bringing camera to you and turning need not be in the same order) **that's why the image is looking flipped *left-right*.**
**why did you turn it that way? why not about horizontal? Had you turned the camera 180 degrees about the horizontal axis we would have seen a vertically inverted photograph of you.**
I hope you understand what turning about horizontal axis means.
When you turn the camera by 180° about horizontal the camera will face the mirror and the camera will be upside down
Remember:
![turning about vertical][1]
turning about vertical
![turning about horizontal][2]
Turning about horizontal
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 looks at it that way but that that's what makes the difference.
- To avoid confusion take another pic with camera facing the mirror and the camera is upside down. i.e turned 180° degrees about horizontal.
What do you see now? If you look at the pic in the camera It's again 'you', beautiful as ever but you 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 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 why the hell am I talking about this camera when we're taking about mirrors?
It's not a camera it's your point of view.
Getting back to the first question
What makes you think that the mirror flipped left and right?
Your took it for granted that your image is not vertically inverted. While taking the second pic I only asked you to make the camera face the mirror,I didn't tell you along which axis it should be rotated but you chose vertical axis only.
You assumed that you are not vertically inverted and asking why you are not vertically inverted.
But 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 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 that some important organs are missing) it would have been easy for you to imagine that your are vertically flipped.
![vertically symmetric human][3]
[1]: https://i.sstatic.net/UMZcX.jpg
[2]: https://i.sstatic.net/Z3K7o.jpg
[3]: https://i.sstatic.net/NMsTG.jpg