While drawing ray diagrams for plane and spherical mirrors, what is generally taken as the point of observation?

Eg, If a concave mirror is presented with, say a wire turned into a triangle, placed from focus towards the mirror, the image can be obtained following rules of reflection but where would the eyes need to be for that particular image to be seen? Like if I were to move around, I'd see different parts of an object. What angle is the images drawn from?

  • $\begingroup$ I don't think I understand the question. The image can be viewed from any location that the reflected rays cover. That is, anywhere your eye can collect reflected light. The entire image will not be in focus and be aberrated in the case of a spherical mirror, but that seems immaterial to your question. $\endgroup$
    – garyp
    Jul 19, 2020 at 12:56
  • $\begingroup$ @garyp What I mean is, right, the image can be viewed from anywhere in the field of view of it, but that would be different (i.e. shifting perspectives) as I move around. So, I could see, more or less, different images from different locations in the field of view. What accounts for that? $\endgroup$
    – Rew
    Jul 19, 2020 at 12:59
  • $\begingroup$ The glib answer is "geometry". So try this geometry experiment. Imagine a line in the object space of a plane mirror, with the line perpendicular to the plane of the mirror. From different vantage points in object space calculate (geometry) what the viewer would see. Do the same thing for a line parallel to the mirror, and a circle parallel to the mirror. $\endgroup$
    – garyp
    Jul 19, 2020 at 13:05
  • $\begingroup$ @garyp can you add a hint as to what I need to he looking for? $\endgroup$
    – Rew
    Jul 19, 2020 at 13:07
  • $\begingroup$ I think what you're interested in is ray tracing. $\endgroup$
    – Ruslan
    Jul 19, 2020 at 13:33

2 Answers 2


We can think of every extended source as a collection of point sources. So, let us only think of point sources. When we draw ray diagrams we often say that an image of a point is formed where 2 rays meet. I was just as baffled as you are but then it clicked of what i think is corect.

Image forming means seeing. Our eyes have evolved to form images of point sources out of diverging rays. (This totally makes sense because you can only have divergent rays from a point source.) The mechanism of our eyes has cornea and lens which converge divergent rays.--so in short we have no problem focusing divergent rays onto our retina.

Here is the trick. If 2 rays are converging then they must be diverging on the other side. So for a 2D scenario, the correct way to see an image is to put your eye behind the image such that the rays enter your eyes. These divergent rays will be converged onto your retina. Done

And for a 3D scenario, a light cone emerges out (for point sources again) so if you manage to get your head such that the cone enters your eyes you will see the image. But again for maximum clarity, put your head on the diverging part of the cone. :-)

Actually, a sharp image is formed if every point forms a point on retina and if every point falls on a region of retina instead of a point, a collection of such points-an extended source becomes blurred.

I cant be sure that my answer is correct as I didn't refer any source.. Require assessment from good physicists.

[![This is for the 2D scenario, apply the same to cone][1]][1]enter image description here

The divergent rays will converge onto the retina by lens. Though only two rays have been shown many rays come out forming the complete image. FYI img is formed between 'C' and 'F'.

  • $\begingroup$ Yeah I understand the cone analogy. The part of the question that "for who are the ray diagrams of spherical mirrors are made" is left unanswered. Where typically is the observing eye placed in a ray diagram i.e. the so-called "image" that is formed, is for the eye that is located where? $\endgroup$
    – Rew
    Jul 19, 2020 at 15:09
  • $\begingroup$ "So for a 2D scenario, the correct way to see an image is to put your eye behind the image such that the rays enter your eyes. " and "But again for maximum clarity, put your head on the diverging part of the cone. :-)" $\endgroup$
    – user248823
    Jul 20, 2020 at 3:01
  • $\begingroup$ meaning the ray diagrams are not universal, right? $\endgroup$
    – Rew
    Jul 20, 2020 at 3:42

In ray tracing, typically light rays leave a point of an object, go through an optical system, and pass through a point of an image.

Sometimes a detector is placed at the image. Different points of the detector see different points of the image. The outcome may be different pixels of a photograph.

In other cases, the light rays keep going past the image. This might be the case for a microsocpe or telescope with eyepieces instead of a camera. You put your eye in the path of the outgoing light. Your eye is a second optical system.

The image point is where light originates from the point of view of your eye. It is the object. Your lens creates an image on your retina.

This just means that you need to do more ray tracing to find the answer. Perhaps make a bigger system that include the mirror and your eye.

  • $\begingroup$ Doesn't specifically answer my question. $\endgroup$
    – Rew
    Jul 19, 2020 at 14:00

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