I have a set of concave/convex mirrors/lenses, and I have drawn out all of the ray tracing diagrams for each possible combination and type of image each optical element can produce.

When I look at the ray tracing diagrams then reproduce the situation in the real world with myself as the "object", all the scenarios make sense to me except one: when I as the object stand beyond the focal point of a concave mirror.

enter image description here

The image ends up as "real" and appears to be in front of the mirror. Now I understand that in the situation above, I as the object would stand at 6m and this would result in the image appearing at 12m. So if I placed a wall or screen at the 12m mark, the image would form on that surface (just like a movie theater screen).... at least I think I interpreted that correctly.

But I still see myself when I look in the mirror?! If the image is supposed to form and come into focus behind me, what in the world am I seeing when looking at the surface of the mirror? I still see myself, just inverted. The surface of the concave mirror still has a reflection on it. It doesn't look quite like a normal flat mirror, in that the image doesn't really look like it's "inside" the mirror, more like it's right on the surface of it.

It's really frustrating because every other possible arrangement of optical elements makes total sense to me, except this one. Any suggestions on how to think about it? Thank you for your time!


1 Answer 1


You are just getting mixed up with what it means for an image to form and what it means to actually see an image with your own eyes.

A good place to start is a flat mirror. What happens if you draw rays like in the image you have posted? You will not find all of the rays converging to a single point, even though you can certainly see something when you look in the mirror. As you mentioned before, we can think of putting a screen at the "image location", but for a flat mirror there is no such place; you could not form an image on a screen. What is going on? Why can you see something from a mirror if there is no image location?

Well, your eyes are lenses, and they form images on your retina. So while there is no image being formed at the location of your eye, the light can be focused to then form an image on your retina.

So, when we talk about real images being formed, we are not talking about the image one would see if their eyes were in that location. We are just talking about a place where light rays from an object converge after interacting with the mirror / lens. If you want to describe what you see through your eyes, you have to add in the additional lenses that make up your eyes.


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