Before starting a detailed tour of physics, I wanted to clear some of my basic doubts.

I have studied the image formation by mirrors and lenses and I'm aware that a real image is formed of an object by a concave mirror and convex mirror. While doing this practically, I did obtained an inverted image on the screen, but along with that I peculiarly saw the image of candle (chosen as object) on the surface of the mirror or lens.

This is quite a usual thing and I also observed or while looking at a steel spoon. But the problem is I cannot identify what that image is. Theoretically the light rays are meeting at some other point but still what is that thing on the surface of mirror or lens ( in case of a concave mirror I found that unknown thing bulged and distorted).

So my question is what is that image on the surface and how it gets formed ?

Please help and thanks ...

  • 1
    $\begingroup$ Possibly a virtual object? (Required to make a real image in case of a convex mirror) Could you try rephrasing your question. It seems a bit vague (to me at least). $\endgroup$ Mar 25, 2017 at 10:00
  • $\begingroup$ Take a steel spoon and look at its inner side and you will see yourself ( a bit, yeah..).I am asking what is the thing you are seeing. It cannot be the image, so then what it is ? $\endgroup$ Mar 25, 2017 at 10:04
  • $\begingroup$ It's a distorted image of you. There are two types of rays (in the extreme): paraxial and marginal. Look them up. Suppose the object is at infinity. What you're seeing is distorted because different light rays from the same point on the object are focussing at different points. $\endgroup$ Mar 25, 2017 at 10:15

1 Answer 1


You are seeing an image of an object in a curved mirror - the spoon.

You may find a soup spoon better than a dessert spoon as it is probably closer to being spherical?

First turn the spoon over so that it curves out towards you and look into it.
You will see an upright and diminished image of yourself in the spoon.
Here is the appropriate ray diagram.

enter image description here

What you will notice is that the image is distorted and this is because the spoon does not have a spherical (actually parabolic) shape and so produces lots of distortion (aberrations).
You will note that the image is formed behind the spoon and so is called a virtual image - one that cannot be projected onto a screen.

The problems with using the spoon are compounded by the fact that it has a very small "radius" of curvature and hence a small focal length.

Now turn the spoon over so that it is boring away from you and so acting like a convex lens.
You will see an inverted and diminished image of yourself which appears to be in the mirror.
It is not.
It is actually a real image which you a focussing on and it is in front of the mirror.

enter image description here

Just because there is no screen there does not mean that the image does not exist.
The image is there in space and your eye can collect the diverging rays from that image and focus them on the retina.

Now bring your face closer to the spoon and the image will get larger but then also fuzzier.
This is because the real image is now inside the near point of your eye, ie less that the least distance of distinct vision for the eye which is roughly 25 cm for the "average" eye.

Your spoon is also a useless shaving mirror.

If your face is within the focal point of the spoon you will get an upright and magnified image.

enter image description here

However the chances are that you will never be able to focus on this image because you eye will have to be very,very close to the mirror and observing the image of your eye will be almost impossible particularly as is inside the near point of your eye. However if you place a pencil close to the mirror you should see a magnified image of the pencil.
For my spoon the pencil had to be about 5 mm from the spoon.

Proper shaving/make-up mirrors have a radius of curvature which might be hundreds of millimetres and so the natural position for the face is within the focal point of the mirror and hence an upright and magnified image is produced.

There was a question which I answered about the magnification produced by a shaving mirror which explains how you can measure the radius of curvature and hence focal length of your spoon.

You may be surprised that you do not need a screen to see a real image. There is an explanation as an answer to a question here.

  • $\begingroup$ This means what I saw on the mirror (spoon), was a real image. But I did the experiment with actual concave mirror and a screen and could see the image on the screen and also the image on the surface. That's what the problem is. Nevertheless you're answered helped me and thanks for that.. $\endgroup$ Mar 25, 2017 at 11:16
  • 1
    $\begingroup$ Try the following. Set up the concave mirror and object so that you get a sharp image on the screen.Now look from behind the screen so that you cannot see the image on the screen. You will see an image in the mirror. Make a mark on the back edge of the screen so that it is touching the image that you can see in the mirror. Move you head up and down and observe the (lack of?) relative movement? between the image and the mark on the back of the screen.If there is no relative movement between them this tells you that they are in a position of no-parallax ie they occupy the same position in space. $\endgroup$
    – Farcher
    Mar 25, 2017 at 12:35

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