The Student Room Group
Reply 1
Without a Shadow
Right I guess this is an easy question but my book has a lot of ambiguity on what the principle focus of the eye is, could somebody simply explain what the principle focus of the eye is?

Thanks


Just where parallel rays of light come together and form an image, the principle focus is just a focus point, at a certain length (focal length) behind and infront of the lens. In the case of the eye, you have a focus point, on the retina of your eye, in order to see. Light diffracts as it enters the eye through the different materials and comes to a point known as the principle focus. And, just ignore him ^
Reply 2
What? If that's sarcasm then I'm embarrassed for you.
Convert light energy to electrical energy?

When you say "principal focus", I'm not really sure what you mean.
Reply 4
Without a Shadow
Right I guess this is an easy question but my book has a lot of ambiguity on what the principle focus of the eye is, could somebody simply explain what the principle focus of the eye is?

Thanks

I think what the OP is asking is by what principle does the eye accomodate for the variations of distance of objects to which it is required to focus light from onto the retina.
Since the eye obviously does not grow longer and shorter to accomodate (like a camera lens does) the eye must obviously use another method.

This is done by the variation of the 'power' of the lens. By increasing the power of the lens, the eye can focus light from a closer source. Conversely by decreasing the power of the lens the eye can focus light from a further source.
Now, you may have noticed that lens that are more powerful tend to be more convex (fatter in the middle). Conversely they tend to be more flat the less powerful they are. The reasoning behind this is beyond the scope of this post, and further information can be obtained by searching for the 'Lens maker equation'.

Now the lens is a soft clear tissue that acts very much like rubber in this case. There are a set of muscles to stretch and to generally deform the lens. Naively one would assume the primary method of deformation would be to stretch out the lens with muscles pulling the lens outwards. However the lens is already normally stretch and flattened because the eye is under internal pressure like a balloon. Instead muscles form a ring around the lens and take the strain off the lens, allowing the lens to un-deform back to a fattened lens. This is why it strains your eye when you look at close objects for a long time. The effect is that your eye muscles are carrying the shopping home.

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