Optical Question!

mothermachine · 7 Sep 2011 at 12:00

Hi folks.

Odd question for you! Hope this makes sense...

Newborn babies can only see around 1 foot in front of them, everything beyond that is blurry.

If you place the baby 1 foot in front of a flat mirror, which is reflecting stuff that is several metres away beyond their face, does the baby see everything in the mirror perfectly sharp/in focus or is everything blurry beyond 1 foot "deep" into the reflection?

I can't quite decide which makes more sense... I would assume that because the entire mirror is in focus, everything which is reflected on it, as a flat plane with no real "depth" to it, is therefore also perfectly in focus. I find it almost impossible to imagine that if one looked at a flat mirror and could only see their face in focus and everything beyond it would be blurry. But then this would be like a portal into the world, where a baby or anyone with extremely short sight can suddenly see everything completely in focus into infinity.

Or, is there some trickery at work that states that a flat mirror does actually have "depth" of some kind, such that everything beyond 1 foot "deep" into the reflection would therefore be blurry.... I'm thinking this must be the answer, as nothing else makes sense.

There is surely a definitive answer to this but I've no way of testing it as I have good eyesight.

Judgeneo · 7 Sep 2011 at 12:03

This, is a very good question. I presume you would just have to be able to focus on the mirror, but don't know for sure, and don't have one handy to experiment

Scrutinize · 7 Sep 2011 at 12:03

rather than explain the physics, I will give you a practical example

I am short sighted, I look into a mirror without my glasses on and the reflection / objects in the distance is blurry. baby will have the same issue.

Judgeneo · 7 Sep 2011 at 12:05

Actually thinking about it, mrk1@1 is right

Reaper 392 · 7 Sep 2011 at 12:05

everything beyond 1 foot total distance would be blurry.

try going very close to a mirror in a decent sized room, holding your finger next to your face and try focusing on the reflection of your finger. you'll see the rest of the room has gone blurry.

without pictures its kinda hard to explain why

mothermachine · 7 Sep 2011 at 12:06

mrk1@1 said:
rather than explain the physics, I will give you a practical example

I am short sighted, I look into a mirror without my glasses on and the reflection / objects in the distance is blurry. baby will have the same issue.

I guess we have our answer! Thanks for confirming.

This raises an interesting idea that mirrors are basically portals, since they don't receive light on a flat (virtually) 2D surface as a projection.

Interesting... my mind still boggles about this. It's hard to imagine that if you're looking at a flat surface of something in perfect focus, what's projected onto it is somehow not necessarily in focus. Weird.

Troll physics at work!

Zefan · 7 Sep 2011 at 12:07

This is actually pretty simple. It's nothing to do with the distance of the mirror at all, as the light still has to travel the same distance and it's this you're seeing - not the object.

mothermachine · 7 Sep 2011 at 12:09

Zefan said:
This is actually pretty simple. It's nothing to do with the distance of the mirror at all, as the light still has to travel the same distance and it's this you're seeing - not the object.

That makes more sense. so the mirror is in focus, but the light that's hitting it is not in focus beyond 1 foot? Becoming clearer (no pun intended..!)

DBT85 · 7 Sep 2011 at 12:10

cannot believe this came up.

The stuff in the mirror is x feet away, the fact that it is being reflected is not even worthy of mentioning and is as relevant as saying "and the child is sitting on a red carpet".

The mirror only reflects light, it doesn't focus it and suddenly make everything pin sharp. If that were the case people wouldn't wear glasses, they'd wear little mirrors.

If the item is 1ft away from the childs eyes then it will see it in focus. That means if you put a child in front of a mirror 1ft away its own reflection will be blurry since the thing that the child is looking at is 2ft away. If your child 1ft vision theory is correct then a child would need to be sitting 6inches from a mirror to see itself in focus.

Belmit · 7 Sep 2011 at 12:11

Imagine a sign on a wall that is just close enough that you can read it. Now imagine the wall is a mirror and you're holding the sign. The sign will appear twice as far away and be unreadable. Same applies to what the baby sees. It's a fascinating topic though.

mothermachine · 7 Sep 2011 at 12:12

Todge said:
cannot believe this came up.

The stuff in the mirror is x feet away, the fact that it is being reflected is not even worthy of mentioning and is as relevant as saying "and the child is sitting on a red carpet".

The mirror only reflects light, it doesn't focus it and suddenly make everything pin sharp. If that were the case people wouldn't wear glasses, they'd wear little mirrors. If the item is 1ft away from the childs eyes then it will see it in focus. That means if you put a child in front of a mirror 1ft away it will be blurry since the thing that the child is looking at is 2ft away.

Thanks for the condescending post.

Anyway, it's not as "derrr" as you think. Think about it - you're looking at a totally flat object that's fully in focus, yet what appears on that surface is not. It's quite a strange phenomenon. Worthy of discussion, I thought...

Zefan · 7 Sep 2011 at 12:12

mothermachine said:
That makes more sense. so the mirror is in focus, but the light that's hitting it is not in focus beyond 1 foot? Becoming clearer (no pun intended..!)

No not beyond 1 foot. There's no actual physical restriction built in to mirrors, they simply reflect all light and we see it. If you're sitting 1 metre away from the mirror looking at yourself then that's the same as focusing on an object 2 metres away from you normally.

There's no sinister trickery at work here!

mothermachine · 7 Sep 2011 at 12:14

Making more sense now... light has to travel once from object to mirror and again from mirror to the eyes. So distance doubles and the strength of the light/photons/whatever halves, making objects in the distance more blurry. I guess this can also work in reverse for people who are near-sighted.

mothermachine · 7 Sep 2011 at 12:15

Zefan said:
No not beyond 1 foot. There's no actual physical restriction built in to mirrors, they simply reflect all light and we see it. If you're sitting 1 metre away from the mirror looking at yourself then that's the same as focusing on an object 2 metres away from you normally.

There's no sinister trickery at work here!

Gotcha! Knew I should have paid more attention in physics class. Shame my teacher was a nasty piece of work. Might have learned these things!

Deadbeat · 7 Sep 2011 at 12:17

mothermachine said:
Thanks for the condescending post.

Anyway, it's not as "derrr" as you think. Think about it - you're looking at a totally flat object that's fully in focus, yet what appears on that surface is not. It's quite a strange phenomenon. Worthy of discussion, I thought...

Condescending post, granted, but it really is that "derr". I can't really imagine spending any amount of time thinking about this issue and not coming tothe conclusion that everything in the mirror would be blurry to the baby. As mentioned, it's the eye that does the focussing, not the mirror. The fact that everything appears fully focused in a mirror is because your eye is focusing the light, not the mirror itself. Belongs in the "stupid questions" thread, tbh

Edit: missed your last post while typing mine. More distant objects do not appear blurry because "the strength of the light/photons/whatever halves", it's because your muscles cannot focus the lens of your eyes sufficiently to bring them into sharper focus.

DBT85 · 7 Sep 2011 at 12:17

mothermachine said:
Thanks for the condescending post.

Anyway, it's not as "derrr" as you think. Think about it - you're looking at a totally flat object that's fully in focus, yet what appears on that surface is not. It's quite a strange phenomenon. Worthy of discussion, I thought...

I just don't get how when you were making this thread the obvious answer didn't occur. Or how it could be classed as "a very good question".

Its up there with the "you wear glasses, what does that sign say over there" nonsense because people for some reason think that glasses = superhuman vision.

Django x2 · 7 Sep 2011 at 12:19

I can't actually believe this is a question worth answering, sorry. A mirror doesn't have "focus". The focus is still done by the eye so what ever is reflected beyond the baby will be out of focus.

mothermachine · 7 Sep 2011 at 12:19

Deadbeat and Todge - I did say that the most likely answer was the one that is true... I wanted confirmation and a nice discussion. And for the most part I got one. Except for you two.... I wasn't the only one who wasn't 100% on this. Anyway thanks to those who took the time to confirm/explain.

DBT85 · 7 Sep 2011 at 12:34

mothermachine said:
Making more sense now... light has to travel once from object to mirror and again from mirror to the eyes. So distance doubles and the strength of the light/photons/whatever halves, making objects in the distance more blurry. I guess this can also work in reverse for people who are near-sighted.

It's nothing to do with the strength of the light. I can see a torch just fine and just as brightly with my specks off as with them on. And Like a baby I cant focus on anything more than about 12" away without my specs on.

An eye works by focussing the light onto your retina. By changing shape and opening or closing the iris your brain is able to look at and focus on any object it wants to. People that need glasses essentially have eyes that are either misshapen slightly or are not as agile as they need to be. What that means is that the light entering the eye is brought to a focal point either before or after the retina. Only if the light is focused ON the retina will you get an image in focus. Wearing glasses refracts the light to compensate.

A result of this is that things will appear different in size to what they actually are. EG. with my glasses on everything looks smaller than it actually is. Since EVERYTHING is smaller you don't actually notice until you wear some contacts which makes the adjustment to the path of the light much closer to the lens in your eye.

mothermachine said:
Deadbeat and Todge - I did say that the most likely answer was the one that is true... I wanted confirmation and a nice discussion. And for the most part I got one. Except for you two.... I wasn't the only one who wasn't 100% on this.

I'm sorry you felt that way. But I'd do it again. Sometimes you have to break some eggs to make a sandwich.

If you ask me why there are no stars in the pictures Nasa took on the moon I'll gladly answer you. If you ask me why wheels are round instead of square though... That's what I saw this as.

FutileBreadMachine · 7 Sep 2011 at 12:45

Haha, "CURE FOR SHORT-SIGHTEDNESS - LOOKING IN A MIRROR". C'mon bro. Evolution in reverse up in here

Optical Question!

Man of Honour