In gravitational lensing, there are three categories of lensing: strong, weak, and micro. As I understand it, strong lensing (just as the name implies) occurs when a source and a gravitational lens are relatively close by and the lens is strong, producing extreme distortions of the light from the source in phenomena such as Einstein rings, weak lensing produces a still distorted image of the source but not as distorted as in strong lensing, and microlensing produces a brightening of an object without any distortion (as described in this question).

My question is, are these three fundamentally different phenomena (perhaps caused by separate terms or different limits in whatever equations govern gravitational lensing) or are they arbitrary classifications in a continuum of gravitational lensing effects, much as infrared, microwave, and radio are arbitrary classifications of the electromagnetic spectrum?

To repeat, my question is not as much about what the differences between these three classifications of gravitational lensing are as much as whether these differences create three largely distinct and independent phenomena or three classifications of the same phenomenon (such as the difference between a lake and a pond, no fundamental difference in the properties of each, just a size difference).


1 Answer 1


They are all the same phenomenon and they are basically just arbitrary distinctions. However they are useful ones.

Strong lensing normally means we see a clear image. We can then use the shape of the image to precisely calculate the mass distribution in whatever is doing the lensing. For strong lensing we need two things (1) the lens must be very massive to produce a big enough image to see, and (2) the alignment needs to be just right i.e. the object must be almost exactly behind the lens.

We get weak lensing when the lens is massive, but there isn't anything exactly behind the lens. In that case the lens produces small changes in the apparent distribution and appearance of objects around it. Again we can use these changes to calculate the mass of the lens, though not as precisely as for strong lensing.

Microlensing is somewhat different. Small objects like stars are very weak lenses and the image they produce is too small to be resolved. However the lensing does cause a measurable change in brightness. So if a star passes in front of some other object we may see the brightness rise then fall again, and it's this phenomenon that is referred to as microlensing.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.