Why can't I focus on a reflection?

Why can't weak-sighted people focus on objects in a nearby mirror?
28 November 2011
Presented by Diana O'Carroll


How a mirror works


For those who are long or short-sighted, have you ever experienced an inability to focus on objects in the mirror - even though you should be close enough to the mirror to see them clearly? Find out why in this QotW. Plus, we ask what's happening when spine-tingling sounds give you the shivers.

In this episode


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

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 it's also how the light is spreading out.

Your eye does exactly the same job focusing on an object that appears 10 metres away in the mirror as focusing on the actual object 10 metres 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 doesn't 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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