<p>Your eye has two functional parts: a lens (that is about ¼cm²) and a light-sensitive surface (the retina) that is covered in rod-cells that detect light. It takes about 10 photons arriving at about the same time (0.1sec) for a rod to react to the light and send a signal to the brain.</p>

<p>This means that, unless about 400 photons arrive from a star onto every square centimetre then there isn't enough light to see the star, even under ideal conditions.  In practice, many more photons are needed for your brain to be able to interpret the signal from your eye as a point of light.</p>

<p>In contrast the mirror of a telescope is much larger. A small amateur telescope could have a mirror that has an area 1000 times larger than the lens of your eye. This means that a star can be 1000 times dimmer, but still be visible when you look at it in the telescope. Large, professional, telescopes have mirrors that are millions of times greater than the area of an eye.</p>

<p>Moreover a telescope can be fitted with a camera, and film (or digital sensors) that are both more sensitive to light than your retina and can integrate the light arriving over a long period of time.  If you make a long exposure, of several minutes, or even hours, then even dimmer objects will become visible.  This can increase the sensitivity by a factor of many thousand again.</p>

<p>The combination of the large light gathering area of the mirror, with the sensitivity and long exposures make possible to "see" things using the mirror of a telescope that can't be seen with the naked eye.</p>

CC BY-SA 4.0
2020-02-23T10:19:13.637Z
Kool beans, I get everything your saying it just seemed to me even with a telescope the size of the moon, to see 31.7 billion light years away-13.7 billion years travelling at 180 000 miles per second is to much to comprehend, to far. But thanks for the idiots guide to astrology 
John
CC BY-SA 4.0
2020-02-23T11:57:14.120Z
@John _astrology_?  
Kirk Woll
CC BY-SA 4.0
2020-02-23T12:16:36.863Z
@John please don't confuse [astronomy](https://en.wikipedia.org/wiki/Astronomy) and [astrology](https://en.wikipedia.org/wiki/Astrology). The former is science, the latter is pseudoscience.
Ruslan
CC BY-SA 4.0
2020-02-23T14:44:18.393Z
Ooh yes, many thanks for the correction on astrology etc. I’m interested in the very big and the very small and everything in between but there’s so much of it. Whilst the answer is understandable it still doesn’t satisfy, if that’s due to ignorance then I accept but a 180,000 miles a second for 13.7 billion years is a long way to see with any aid. Could you tell me why it is assumed the particle prior to the Big Bang was smaller than a grain of sand and it wasn’t some huge object that blew up to leave all the debris we see today, for sure we know how planets etc are formed today, indisputable
John
CC BY-SA 4.0
2020-02-23T14:44:18.393Z
but for what reason all this came from something smaller than a grain of sand and if the theory of multiverses is a possibility what then would divide or separate them ??? The most common word I hear when listening to documentaries is “could” “should “ might” and “perhaps” but what we do know and can see is phenomenal.
John
CC BY-SA 4.0
2020-02-23T18:48:03.423Z
In @John's defense, you'd think the "olgoy" would be the one that is actually science... =(
corsiKa
CC BY-SA 4.0
2020-02-24T00:17:27.837Z
@corsiKa That's marketing for you. Never trust the marketing.
DKNguyen
CC BY-SA 4.0
2020-02-24T00:19:21.523Z
@corsiKa Indeed, if you read it literally, astronomy and astrology are translated as "the practise of naming stars" and "the practise of talking about stars".
Ingolifs
CC BY-SA 4.0
2020-02-24T01:14:28.637Z
@Ingolifs So TMZ should really be Astronomy Daily? :-p
corsiKa
CC BY-SA 4.0
2020-02-24T11:03:22.040Z
@Ingolifs: No that's wrong. The "-nomy" suffix comes from "-νομια" which is a Greek suffix derived from "νομος" which means "law". Not related to "ονομα" (name). Astronomy comes from "[αστρονομια](https://en.wiktionary.org/wiki/ἀστρονομία)" from "αστρον+νομος" (ending changed to "-ια" because astronomy was classified as feminine).
user21820
CC BY-SA 4.0
2020-02-24T11:07:42.877Z
@corsiKa: See my above comment. It actually makes sense that astronomy is the science, since it is about the law[s] of stars. Though it means we should be studying bionomy.
user21820
CC BY-SA 4.0
2020-02-24T13:23:12.777Z
@John This is getting very broad. But to keep things simple - we don't resolve light from a single star 13.7 billion years ago. We see entire galaxies and clusters of galaxies. If four hundred photons reach us from such a great distance, it's not enough to show how many stars you're watching - but you still see a bright spot. You're probably referring to images like the Hubble Deep Field - all of those dots are either stars in our galaxy (or neighboring galaxies), or entire galaxies really far away. Later, the UDF (and XDF) was taken over _months_ of exposure and still only shows galaxies.
Luaan
CC BY-SA 4.0
2020-02-22T10:48:07.717Z
2020-02-22T10:48:07.717Z
James K
<p>How does a mirror pick up light from vast distances away? And, if that light can reach a mirror, why can’t we see that light?</p>

CC BY-SA 4.0
2020-02-22T07:44:14.847Z
Did you have a particular mirror in mind? One that was much, much bigger than your eye for example?
ProfRob
CC BY-SA 4.0
2020-02-23T00:46:57.587Z
this question confused me at first, because technically it's not the mirror of a telescope that does the work - in fact, a telescope *could* consist solely of glass without any mirrors.
Michael
CC BY-SA 4.0
2020-02-24T14:18:02.707Z
@Michael It can also consist of [mirrors without any glass](https://en.wikipedia.org/wiki/Reflecting_telescope)...
J...
<observational-astronomy>
How can a mirror see things that the naked eye can’t?
John
# How can a mirror see things that the naked eye can’t?

(Asked by: username_0 on 2020-02-22)

How does a mirror pick up light from vast distances away? And, if that light can reach a mirror, why can’t we see that light?

**Tags:** observational-astronomy
> **Comments:**
> **username_1**: Did you have a particular mirror in mind? One that was much, much bigger than your eye for example?  
> **username_2**: this question confused me at first, because technically it's not the mirror of a telescope that does the work - in fact, a telescope *could* consist solely of glass without any mirrors.  
> **username_3**: @Michael It can also consist of [mirrors without any glass](https://en.wikipedia.org/wiki/Reflecting_telescope)...  

---

## 1 answers

### Answer by username_4 on 2020-02-22. Score: 44

Your eye has two functional parts: a lens (that is about ¼cm²) and a light-sensitive surface (the retina) that is covered in rod-cells that detect light. It takes about 10 photons arriving at about the same time (0.1sec) for a rod to react to the light and send a signal to the brain.

This means that, unless about 400 photons arrive from a star onto every square centimetre then there isn't enough light to see the star, even under ideal conditions. In practice, many more photons are needed for your brain to be able to interpret the signal from your eye as a point of light.

In contrast the mirror of a telescope is much larger. A small amateur telescope could have a mirror that has an area 1000 times larger than the lens of your eye. This means that a star can be 1000 times dimmer, but still be visible when you look at it in the telescope. Large, professional, telescopes have mirrors that are millions of times greater than the area of an eye.

Moreover a telescope can be fitted with a camera, and film (or digital sensors) that are both more sensitive to light than your retina and can integrate the light arriving over a long period of time. If you make a long exposure, of several minutes, or even hours, then even dimmer objects will become visible. This can increase the sensitivity by a factor of many thousand again.

The combination of the large light gathering area of the mirror, with the sensitivity and long exposures make possible to "see" things using the mirror of a telescope that can't be seen with the naked eye.

> **Comments:**
> **username_0**: Kool beans, I get everything your saying it just seemed to me even with a telescope the size of the moon, to see 31.7 billion light years away-13.7 billion years travelling at 180 000 miles per second is to much to comprehend, to far. But thanks for the idiots guide to astrology   
> **username_5**: @John _astrology_?    
> **username_6**: @John please don't confuse [astronomy](https://en.wikipedia.org/wiki/Astronomy) and [astrology](https://en.wikipedia.org/wiki/Astrology). The former is science, the latter is pseudoscience.  
> **username_0**: Ooh yes, many thanks for the correction on astrology etc. I’m interested in the very big and the very small and everything in between but there’s so much of it. Whilst the answer is understandable it still doesn’t satisfy, if that’s due to ignorance then I accept but a 180,000 miles a second for 13.7 billion years is a long way to see with any aid. Could you tell me why it is assumed the particle prior to the Big Bang was smaller than a grain of sand and it wasn’t some huge object that blew up to leave all the debris we see today, for sure we know how planets etc are formed today, indisputable  
> **username_0**: but for what reason all this came from something smaller than a grain of sand and if the theory of multiverses is a possibility what then would divide or separate them ??? The most common word I hear when listening to documentaries is “could” “should “ might” and “perhaps” but what we do know and can see is phenomenal.  
> **username_7**: In @John's defense, you'd think the "olgoy" would be the one that is actually science... =(  
> **username_8**: @corsiKa That's marketing for you. Never trust the marketing.  
> **username_9**: @corsiKa Indeed, if you read it literally, astronomy and astrology are translated as "the practise of naming stars" and "the practise of talking about stars".  
> **username_7**: @Ingolifs So TMZ should really be Astronomy Daily? :-p  
> **username_10**: @Ingolifs: No that's wrong. The "-nomy" suffix comes from "-νομια" which is a Greek suffix derived from "νομος" which means "law". Not related to "ονομα" (name). Astronomy comes from "[αστρονομια](https://en.wiktionary.org/wiki/ἀστρονομία)" from "αστρον+νομος" (ending changed to "-ια" because astronomy was classified as feminine).  
> **username_10**: @corsiKa: See my above comment. It actually makes sense that astronomy is the science, since it is about the law[s] of stars. Though it means we should be studying bionomy.  
> **username_11**: @John This is getting very broad. But to keep things simple - we don't resolve light from a single star 13.7 billion years ago. We see entire galaxies and clusters of galaxies. If four hundred photons reach us from such a great distance, it's not enough to show how many stars you're watching - but you still see a bright spot. You're probably referring to images like the Hubble Deep Field - all of those dots are either stars in our galaxy (or neighboring galaxies), or entire galaxies really far away. Later, the UDF (and XDF) was taken over _months_ of exposure and still only shows galaxies.  

---

