Strong gravitational lensing produces multiple images if the object lens and observer are in a specific alignment. This is due to there being multiple stationary points in the time arrival surface for the paths the light can take. So far I understand that.

Now what is puzzling me for a while is what prevents a classical lens, for lack of better term, from displaying the same behaviour? Is it just a lack of scale? In gravitational lenses we are dealing with massive scales so the speed of light comes into play where as your run of the mill spy glass doesn't really care about that.

So I guess what I'm asking is if I had a galaxy sized massless glass lens would it look the same as a gravitational lens?

  • $\begingroup$ Glass lenses are usually lacking errors which distort the image. Have a look at this answer. $\endgroup$
    – A. P.
    Jan 18, 2021 at 12:39

1 Answer 1


[W]hat prevents a classical lens, for lack of better term, from displaying the same behaviour?

Classical lenses are designed to produce a single image under normal circumstances.

For a gravitational lens corresponding to a point mass the angle of ray deflection increases the closer it passes to the center, while for a “classical lens” the angle of deflection is proportional (at least at a first approximation) to the distance from the center.

Are multiple images an exclusive property of gravitational lenses?

No. It is possible to replicate optical properties of gravitational lenses by a suitably deformed glass of water surfaces.

For example, the paper

uses menisci formed on water surface near drawing pins slightly lifted above the water level to replicate many features of gravitational lensing, including multiple images:

Image from M. Selmke page

  • $\begingroup$ I think the key property (for glass lens) is that one or both surfaces must demonstrate changes in the sign of curvature across the surface. Otherwise you can get blurring and distortion but not multiple images. $\endgroup$ Jan 18, 2021 at 15:17

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