# The eye-Biology GCSE watch

1. Why does the lens become flattened when looking at distant objects. I know this makes it refract less light but how would that help?
2. To see something in focus the light being emitted off the object must be focussed on the fovea at the back of the eye. Light rays coming from a distant object reach the eye at an almost horizontal angle, the cornea is naturally curved so it refracts the light slightly which is enough for it to be at the correct angle to reach the fovea. If the lens didn't flatten and remained at a more convex shape then the light would be refracted too much and not focus onto the fovea, it would reach the back of the eye at the wrong place causing an unfocussed image to be seen.

In your question above, "I know this makes it refract less light...", it doesn't refract less light, it refracts the same amount of light but at a less steep/sharp angle.
3. Due to the object’s distance and the rays being parallel to the lens, the lens flattens to refract light less. This makes it focus exactly on the retina.
4. (Original post by SmilingWombat)
To see something in focus the light being emitted off the object must be focussed on the fovea at the back of the eye. Light rays coming from a distant object reach the eye at an almost horizontal angle, the cornea is naturally curved so it refracts the light slightly which is enough for it to be at the correct angle to reach the fovea. If the lens didn't flatten and remained at a more convex shape then the light would be refracted too much and not focus onto the fovea, it would reach the back of the eye at the wrong place causing an unfocussed image to be seen.

In your question above, "I know this makes it refract less light...", it doesn't refract less light, it refracts the same amount of light but at a less steep/sharp angle.
What about when looking at close objects then why do we need more refraction?
5. (Original post by HN786)
Why does the lens become flattened when looking at distant objects. I know this makes it refract less light but how would that help?
Moved to the Biology, biochemistry and other life sciences study help forum
6. (Original post by HN786)
What about when looking at close objects then why do we need more refraction?
As the rays are more spread out when they enter the lens, so they need to be refracted more in order to focus directly on the retina. Learn the two images I used, as they often come up in exams.
7. (Original post by HN786)
What about when looking at close objects then why do we need more refraction?
Light rays coming from an object close to us are more diverged or spread out (as opposed to light rays from a distant object) when they reach the eye. The cornea refracts the light rays a little but not enough to have them land on the fovea. The lens inside the eye needs to be convex (rounded) to refract the light rays further so that they land exactly on the fovea so that we can interpret the image in focus.

Look at the illustration I have linked and look at the top illustration first. Here the lens is convex (rounded), it's accommodating for a close object, this whole process is called "accommodation" by the way. The blue lines show how the light is refracted and lands perfectly on the fovea. The yellow lines show what is happening to light rays from a distant object, they are refracted too much and cross over inside the eye landing away from the fovea, therefore interpreted as an out of focus image by the brain. Look at something close to you, it's in focus, without looking away from the closeby object you'll notice objects further away (in your peripheral vision) are out of focus. This is the reason why. The bottom illustration shows the opposite, how the eye is set to focus on a distant object and what is happening to light rays coming from a nearby object.

http://www.nylaservision.com/wp-cont...comodation.png
8. This video might be useful too, specifically from 4:20 if you want to jump straight to the part where he explains accommodation.

https://youtu.be/eRShDdGucJw

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