<p>Gravitational lensing is just geometry, as is optical lensing. The amplification factor describes the increase of the <strong>area</strong> of the image (at constant surface brightness). </p>

<p>BTW, the amplification factor <em>does</em> depend on the mass $M$ of the lens, since $u=\theta/\theta_E$ and $\theta_E$ depends on $M$. Here $\theta$ and $\theta_E$ are the relative angular distance between source and lens and the radius (angle) of the Einstein ring for the lens, respectively.</p>

CC BY-SA 3.0
2017-03-03T23:32:45.867Z
I think it is the second part here that answers the question. You could explain what theta and theta_E represent. A picture of the geometry would be even better.
James K
CC BY-SA 3.0
2017-03-03T08:28:47.313Z
2017-03-04T08:07:55.617Z
2017-03-04T08:07:55.617Z
Walter
<p>At first I assumed the amplification factor meant an increase in brightness. But since the equation: $$A(u)=\frac{u^2+2}{u\sqrt{u^2+4}}$$ is independent of mass, this make no sense to me. Wouldn't everything be infinity magnified (if its mass independent empty space should work, right)? Even if its simply not valid for a zero mass, everything would be amplified by an arbitrarily high amount any time even a neutrino passes arbitrarily close to the right $u$?
But if its not brightness, I don't know what signal is being amplified.</p>

CC BY-SA 3.0
2017-03-03T14:38:00.637Z
I think the Wikipedia page explains this pretty clearly
Carl Witthoft
<gravitational-lensing>
What does the 'amplification factor' of gravitational microlensing mean?
mjgreen
# What does the 'amplification factor' of gravitational microlensing mean?

(Asked by: username_0 on 2017-03-03)

At first I assumed the amplification factor meant an increase in brightness. But since the equation: $$A(u)=\frac{u^2+2}{u\sqrt{u^2+4}}$$ is independent of mass, this make no sense to me. Wouldn't everything be infinity magnified (if its mass independent empty space should work, right)? Even if its simply not valid for a zero mass, everything would be amplified by an arbitrarily high amount any time even a neutrino passes arbitrarily close to the right $u$?
But if its not brightness, I don't know what signal is being amplified.

**Tags:** gravitational-lensing

> **Comments:**
> **username_1**: I think the Wikipedia page explains this pretty clearly  

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## 1 answers

### Answer by username_2 on 2017-03-03. Score: 6 | Accepted answer

Gravitational lensing is just geometry, as is optical lensing. The amplification factor describes the increase of the **area** of the image (at constant surface brightness).

BTW, the amplification factor *does* depend on the mass $M$ of the lens, since $u=\theta/\theta\_E$ and $\theta\_E$ depends on $M$. Here $\theta$ and $\theta\_E$ are the relative angular distance between source and lens and the radius (angle) of the Einstein ring for the lens, respectively.

> **Comments:**
> **username_3**: I think it is the second part here that answers the question. You could explain what theta and theta_E represent. A picture of the geometry would be even better.  

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