Reading glasses

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 MG 07 Aug 2022

Tried some today. Cor!

But how do they work, or in fact any lenses? Diagrams like this suggest an in focus lens focuses everything to a dot, which is clearly wrong. Somewhat does in focus mean?


 lithos 07 Aug 2022
In reply to MG:

the point, or in 2d, a plane is focused onto your retina (back of the eye).  As you get older your lens isnt able to flex enough (squidge up) to make that focus onto your retina so the glasses help a bit.

see the first picture here https://en.wikipedia.org/wiki/Focus_(optics)  of an object  A-B being 'projected' onto the retina. you want it to be in focus.meaning all the rays from that one point (eg point A) cross/meet on the retina not in front or behind it. similar for point B and all the other points in between.

https://www.bbc.co.uk/bitesize/guides/zq7thyc/revision/5 is a good intro

Post edited at 20:00
OP MG 07 Aug 2022
In reply to lithos:

Thanks. The wiki diagram seems much clearer. Is it correct to say that the focal point isn't where the retina is but in front of it then?

 lithos 08 Aug 2022
In reply to MG:

no it should be on the retina.  the focal point moves as the lens changes shape to focus on near things vs far things.  far things need the light to be bent less (the rays will be like in your first picture near parallel)  near things the rays are more angled and need bending more (a fatter lens)

always trying to focus (have the focal point on the retina)

 BruceM 11 Aug 2022
In reply to MG:

I like to think about how a pinhole camera works (directing points of light out in space to a specific corresponding position on the imaging surface) and then think of a lens as a craftily shaped piece of glass that implements the idea of a pinhole, but does it over a wider cross-section (aperture) so as to collect more light and make a brighter image.

Edit: oh yeah, but the point is that the crafty lens only works like a super light-collecting pinhole for light from things - and to an image plane - at a certain (focal) distance. 

Post edited at 08:39
 Toerag 11 Aug 2022

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