Einstein ring explained (IMAGE)
Caption
When we observe a distant galaxy with our telescope, its light may encounter another galaxy on its way to us. The foreground galaxy acts like a magnifying lens, bending the travelling light rays due to its gravity. This is called gravitational lensing. If the background galaxy, the lensing galaxy, and the telescope are perfectly aligned, the image appears as a ring – called an Einstein ring. Einstein rings were first theorised to exist by Einstein in his general theory of relativity.
Credit
ESA
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