<p>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.</p>

<p>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.)</p>

CC BY-SA 4.0
2019-01-25T07:56:41.667Z
a tiny bit like [this](https://commons.wikimedia.org/wiki/File:Great_Barracuda,_corals,_sea_urchin_and_Caustic_(optics)_in_Kona,_Hawaii_2009.jpg)
uhoh
CC BY-SA 4.0
2019-01-25T19:14:26.667Z
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.
Carl Witthoft
CC BY-SA 4.0
2019-01-25T07:33:17.883Z
2019-01-25T07:33:17.883Z
James K
<p>How can gravitational lensing makes a quasar brighter than it would in the absence of a foreground galaxy?</p>

CC BY-SA 4.0
2019-01-25T15:16:13.060Z
How can glass lensing make sunlight appear intense enough to start fires?
Mason Wheeler
CC BY-SA 4.0
2019-01-25T19:49:34.077Z
How does a telescope lens make dim stars bright enough to see?
Barmar
<cosmology><observational-astronomy><gravitational-lensing><quasars>
How can gravitational lensing makes a quasar appear brighter?
Haris Ansari
# How can gravitational lensing makes a quasar appear brighter?

(Asked by: username_0 on 2019-01-25)

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

**Tags:** cosmology, observational-astronomy, gravitational-lensing, quasars

> **Comments:**
> **username_1**: How can glass lensing make sunlight appear intense enough to start fires?  
> **username_2**: How does a telescope lens make dim stars bright enough to see?  

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## 1 answers

### Answer by username_3 on 2019-01-25. Score: 16

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.)

> **Comments:**
> **username_4**: a tiny bit like [this](https://commons.wikimedia.org/wiki/File:Great_Barracuda,_corals,_sea_urchin_and_Caustic_(optics)_in_Kona,_Hawaii_2009.jpg)  
> **username_5**: 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.  

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