 So the absolute simplest piece of optics that we build is typically a mirror. So there's a mirror It's got a silvered surface and here's us Now if we point our eyes in the direction of that mirror the light that's reaching our eyes is going to be bouncing off our Mirror at a particular angle and we know that the angle that it bounces off the mirror is going to be same as the angle That came into the mirror on so if I've got light That's got to get from my feet up to my eyes through the mirror that I know that's got to get exactly Halfway up to the mirror and then exactly the same distance back up to my eyes I haven't drawn this perfectly But we know that reflection means at this angle the angle of incidence and this angle the angular reflection are the same and If those two angles are the same if I consider this triangle here Sort of the mirror up to the eyes and the triangle that goes from the mirror down to the feet Then those two triangles because they share that same length in the middle and because those two angles are the same It means those two triangles are exactly the same shape just mirror reversed And that's why it's equal distances from the feet up to the mirror as it is from the mirror up to the eyes And exactly the same thing happens when you consider the light reaching from the top of the head to the eyes Now the eyes think that light just traveling as it normally does in straight lines And so it thinks that those feet came from this direction and that they were about That far away from various other clues. I think that the top of the head came from this direction And it's about that far away and so on for each other part of the body and so the eyes see something over here And that's why we see an image inside the mirror It looks though It's actually coming from a room and a person inside that mirror and this is called a virtual image Now if you want to find out exactly where that image is for a flat mirror You just draw those light rays like that and you do some geometry We note that the angle over here is exactly the same as the angle over here because that's just a straight line And therefore we can tell that this triangle here and this triangle here must be just copies of each other Except upside down So then we can tell that image basically looks exactly the same height as us We know that this height the height of our object which in this case is us and the height of our image is the same So those two heights are the same and we also know that the distance between the mirror and our object Is the same as the distance between the mirror and our image So flat mirrors are pretty simple