Question of the Week

Why couldn't I focus on a reflection when close to the mirror?

Sun, 27th Nov 2011

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Donna Colson asked:

While getting ready for work one morning I encountered something I can't figure out.


I wear glasses and without them objects are blurry unless I get very close, within a few inches.† I always take my glasses off when I blow dry my hair in the morning.† While standing in front of the mirror in my bathroom I can see a reflection of the tv screen in the bedroom.† It is blurry when my glasses are off and much clearer when I put my glasses on.† One morning I was interested in a show on tv whilst I was drying my hair so I figured I could watch what was going on if i put my face very close to the mirror while I dried my hair.† I tried but no matter how close I got to the mirror the reflected image of the tv stayed blurry.† I was close enough to where I normally can see an object clearly without my glasses.† I can't figure out why the reflection remained blurry despite my closeness to the mirror. Why would that be?


Thanks in advance!!


So even though the TV ought to be close enough, why does the mirror keep it blurry?

We posed this question to Dr Brian Robertson, research associate at the photonics and sensors group in the Department of Engineering, University of Cambridge...

Brian - In answer to your question, you have to focus a camera or your eyes because as your light leaves an object it spreads out but to get a good image, the lenses in your eye have to bend all the light from one point in the object onto one point on the retina in the back of your eye.  If you're close to an object, light is spreading out more quickly so the light needs more bending to produce a sharp image and come into focus than if you're far away.  The eye adjusts to these distances by changing the shape of the lens - short and long sightedness occurs when the eye is less able to accommodate these changes.

Light reflecting from a mirror gives the impression that the object is behind the mirror and you get this impression because the light having reflected from the mirror is moving exactly as if it were coming from an object behind the mirror.  This is not just true of the direction of the light is travelling, but itís also how the light is spreading out.  So your eye does exactly the same job focusing on an object that appears 10 meters away in the mirror as focusing on the actual object 10 meters away.  So, if you're short sighted, the object in this case, the television screen, still appears blurred regardless of how close you are to the mirror.

Diana -   On the forum, RD said that it doesnít matter how close you get to the mirror when an object is a certain distance from it.  If you're short sighted then the lens in your eye cannot adjust for the amount of spread the light has taken over that distance.  He also mentioned that a convex mirror can cause the object to appear even farther away as it spreads the light even more.  Again, making it blurry if you're short sighted.  Next week, nails on a blackboard.


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The optical path of objects seen in a mirror is observer-mirror distance plus mirror-object distance, not just observer-mirror distance.

If you are short-sighted looking at distant things via a (flat*) mirror held close to your eye won't render them sharp: their reflected image is not in the plane of the mirror. 

mirror was convex rather than flat, (i.e. bulged outwards, opposite to a make-up/shaving mirror),
it could correct for myopia and render a reflection of the distant TV sharp to your myopic eye].
RD, Sun, 20th Nov 2011

This is exactly what I think. No matter how far you are from the mirror, the distance of the mirror reflection to the object will never change so it wouldn't make a difference.

I think... Rawsko, Mon, 21st Nov 2011

What about if mirrors are reflecting light from another mirror. Would the image still be blurry? demalo, Wed, 14th Dec 2011

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