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How can gravitational lensing makes a quasar brighter than it would in the absence of a foreground galaxy?

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    $\begingroup$ How can glass lensing make sunlight appear intense enough to start fires? $\endgroup$
    – Mason Wheeler
    Commented Jan 25, 2019 at 15:16
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    $\begingroup$ How does a telescope lens make dim stars bright enough to see? $\endgroup$
    – Barmar
    Commented Jan 25, 2019 at 19:49

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The quasar gives out light in all directions. The light spreads out in space. Only a very small amount of that light would be pointed exactly in the direction of your telescope.

But if a large galaxy or galaxy cluster is between the quasar and us, it bends some of the light towards us, making the quasar brighter (it would also distort the shape, but quasars are too small for their shape to be seen) This makes it brighter for us (and dimmer in other parts of space which would have got the light that is now bent towards us.)

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    $\begingroup$ a tiny bit like this $\endgroup$
    – uhoh
    Commented Jan 25, 2019 at 7:56
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    $\begingroup$ To be clear: nothing can make the image brighter than the quasar itself is. That's that optical "entendue law". What happens is the image is made brighter than it would have been sans the lensing. $\endgroup$
    – Carl Witthoft
    Commented Jan 25, 2019 at 19:14

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