Many people should have realised, when looking into a concave curved mirror (or even a rather reflective spoon in that fact) at a close distance, you will see a slightly distorted reflection. But as you move further away, the image will suddenly become upside-down.

why is this?


When you look into a concave mirror you're looking at an image of yourself. The position of the image is described in this article on the mirror equation.

When you are farther away from the mirror than the focal point a real image is formed between the mirror and it's focal point. This image is inverted. When you get closer to the mirror than the focal point a virtual image is formed behind the mirror and this image is not inverted. That's why the image flips as you get closer. You'll also find the real image is smaller than the object while the virtual image is bigger than the object.

A quick Google found this article that shows the ray diagrams for real and virtual images.

Beginners to optics find the idea of virtual images confusing. A real image can be seen on a screen i.e. if you put a piece of paper in the position of the image you'll see the image on the paper. With a virtual image the light rays never come to a focus so there is no place you can put a piece of paper to see the image. However your eye contains a lens and can bring the diverging light rays to a focus on your retina. Hence your eye can see a virtual image even though it couldn't be projected onto the sheet of paper.

  • $\begingroup$ If the mirror itself was shrunk proportionally and moved forward and back within the geometric cone whose focal point you describe, that would yield the virtual image at every point, correct? $\endgroup$ – bright-star Oct 24 '16 at 4:48
  • $\begingroup$ @TrevorAlexander: I'm not sure what you are asking. You could post a new question or ask in the chat room. $\endgroup$ – John Rennie Oct 24 '16 at 5:41

You might notice when looking at yourself in a concave mirror that your image flips not at the focal point,f, but at the radius of curvature of the mirror, R. R=2f.

Applying the lens equation one determines that the image of an object (your face for example) should flip at the focal point. So what gives?

The answer is that the mirror and the lens in your eye act together kind of like a compound telescope. When your face is at a distance f < "face" < R the image of your face created by the mirror is behind your head and inverted. The lens in your eye focuses this virtual object onto the back of your eye and the image is not inverted (usually the convex lens in your eye inverts a real image). Because your eye is used to seeing real image - the image of your face appears upright.


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