Do babies see mirrors as focused or blurry?

Submitted by Lord_Poopsicle t3_zp2yxy in askscience

A quick search shows that a 2 month old baby can see around 12 inches away. What happens when they look at a mirror within that range? Can they see things across the room clearly, because the mirror itself is less than a foot away? Do things in the mirror look just as blurry as they would normally, since the object in the mirror is more than a foot away?

I noticed while changing my kid's diaper that he would stare really intently into the mirror on the wall just above the changing table. Sometimes it seemed like he was accurately tracking my wife moving around the room - through the mirror - even though she was way outside the normal limit of his vision. It could be that he was just tracking a sort of blobby movement (which he does a lot of anyway.) But his level of interest in the mirror is really intense.

(To add, because the mirror is so close and above him, I think he's able to see a lot of the room well, but not much of himself. I don't think he's just fascinated by his reflection, but that is obviously another real possibility)

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BeneficialWarrant t1_j0ru3a1 wrote

If you stand 12 inches in front of a mirror and focus on your own reflection, you are focusing your eyes to resolve an image that is 24" away. You can verify this by smearing a piece of toothpaste on your bathroom mirror. Focus on the toothpaste and your reflection will be out of focus.

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bigbluedoor t1_j0sn54g wrote

eye exams at doctors offices often use mirrors to simulate further distance in a small office

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doc_nano t1_j0sikiu wrote

This is correct. Another good example is trying to read text across the room through a planar mirror if you’re nearsighted. Let’s say you can read text clearly without glasses if it’s a foot away but not if it’s 20 feet away. If you’re standing 1 foot from a mirror looking at text 20 feet behind you, light from the text will have to travel about 22 feet before it reaches your eyes, and you won’t be able to read it without glasses because you’re effectively trying to look at text that is 22 feet away, not 1 foot away.

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esach88 t1_j0tw0q8 wrote

Interesting, could mirrors be used to help prevent eye issues from constant near focus? Eg. Needing glasses due to always reading books and using screens and staying indoors.

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BeneficialWarrant t1_j0tzfdh wrote

There isn't any evidence that looking at close up objects causes any permanent harm to eyes.

Losing the ability to focus on close up objects with aging (presbyopia) is just kinda one of those things that happens. The lens slowly becomes less stretchy (elastic) and won't fully snap back into a round shape when tension on it is released.

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rootofallworlds t1_j0utwve wrote

If you want to put the book or screen ten times further away, the letters need to be ten times the size to have the same angular size. That applies whether or not you put flat mirrors you put in the light path.

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