I've seen Youtube videos explaining how to "safely" observe the sun by projecting an image of the sun onto a piece of cardboard behind the eyepiece of a refracting telescope or binoculars. However, I don't understand how that works: The image created by the eyepiece is a virtual image that is typically brought into focus on the retina through the lens of the eye. If we omit the lens in the eye, there should be no image that can be projected. But in the videos it seem to work. Why is that?

  • 2
    $\begingroup$ Cannot write a good answer, but the light rays are not virtual. You can even use a pinhole camera (see also camera obscura) to observe the sun, without any lense. $\endgroup$
    – DarkDust
    Commented Oct 25, 2023 at 9:20
  • $\begingroup$ Also... both refracting telescopes and binoculars typically contain lenses. $\endgroup$
    – notovny
    Commented Oct 25, 2023 at 9:55
  • $\begingroup$ @DarkDust, the pinhole model isn't a good analogy because it doesn't contain an eyepiece. Instead it has a view screen where the eyepiece would be in a telescope. $\endgroup$ Commented Oct 25, 2023 at 15:26

1 Answer 1


I thought some more and came up with the following answer:

The eypepiece is used as a magnification glass when actually placing the eye against the the eyepiece: In this case, we pull the focus so the real image created by the objective is created somewhere inside of the focal length of the eyepiece, thereby creating a virtual, enlarged image between the object being viewed and its real image.

However, when projecting the sun on a screen, the eypiece is pulled so far away from the real image that its distance from the eyepiece exceeds the focal length of the eyepiece. In this case, the rays entering the eyepiece will converge at some distance from the eyepiece outside of the telescope, forming a second real image that can then be captured by a screen.The eyepiece is "abused" as a projector by moving it out of the range where it creates a virtual image. In this case, if you were to place your eye against the eyepiece (which you most certainly shouldn't do when observing the sun!!), you would only see an out of focus image, as the eyepiece is not working as a magnifying glass. So focussing the image when in "eypiece" mode is completely different from focussing the image when in projection mode.


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