Gravitational Lensing

Gravitational lensing occurs when a large galaxy lies between the Earth and some distant object of interest. As this light moves away from the object in the rough direction of the earth, some of it might pass close to the intervening galaxy in such a way that the light is deflected on a path towards the Earth. The intervening galaxy is acting like a lens to focus the light rays from the distant object e.g. a quasar. There is usually more than one possible path for the light, and it can be focused to form several different images. If the galaxy is on the exact line of sight, all the images appear at the same distance.

If the massive galaxy is off-center (as might be expected) with respect to the line between the quasar and the Earth, then the two light paths would travel different distances around the galaxy. and would appear to be non symmetrical.

Since the distances between each galaxy, object and Earth is much larger than the sizes of any of them, we can treat galaxy and object as point masses, allowing calculation of the distance to the object in question.

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