GCSE Biology - can anyone explain the eye to me?

Can someone explain the eye to me (for gcse aqa biology)? I don't get how the eye accomodates to near/ far objects or how glasses/ lenses work to fix long sightedness/ short sightedness... thank you!

Reply 1

princesschip

Original post by VaBook7

Can someone explain the eye to me (for gcse aqa biology)? I don't get how the eye accomodates to near/ far objects or how glasses/ lenses work to fix long sightedness/ short sightedness... thank you!

Hi! I'm also taking aqa gcse bio and here's what we've got on the eye: so essentially the lens is suspended behind the pupil by suspensory ligaments (like little strings attached around the edges of the circular lens). The other end of each suspensory ligament is attached to the ciliary muscle which is a round muscle like a ring. When looking at a close object, light rays from the object enter the eye at an angle, and the ciliary muscle contracts. When muscles contract they get shorter, so the circle gets smaller and the suspensory ligaments go slack. The ligaments aren't pulling on the lens so it is thick and round, so the light which passes through it is refracted more, which means even though it entered at an angle the lens can still focus the light properly onto the retina, preferable the fovea (yellow spot). If the object is further away, the light rays enter the eye pretty much straight on. The ciliary muscle relaxes, getting longer and so widening the ring. This pulls those suspensory ligaments taught so they are pulling the lens outwards in all directions. It gets stretched, becoming flatter and thinner, so light is refracted less when passing through it and can be focused onto the retina (should be able to find some good diagrams online of the light angles and refraction). If you are short-sighted (myopia), you can't see near objects clearly as the lens does not refract the light enough, and the light rays are not focused on a point by the time they reach the retina, making it blurry. To fix it you can use a convex (converging) lens since it does some of the refraction before light even enters the eye, making the rays enter in a more 'straight on' direction, so they don't need to be refracted as much to focus. The opposite is true for hyperopia (long sightedness), where the rays are refracted too much by the lens when looking at far objects, so they are focused before reaching the retina, which then cannot produce a clear image. The diverging (concave) lens refracts the light rays so that they enter the eye at an angle similar to how light from a near object usually would, so the extra refraction from the lens focuses the light correctly onto the retina. I know that was really long winded but I hope it helps!