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AQA Biology Direction of Light??

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Why is the light pointing that way? Surely it's meant to go through the light sensitive cells first ??
Anyoneee
It is shown that way round because - bizarrely - the anatomy of the retina is actually back-to-front in its structure. It's one of those quirks that shows us how the eye evolved - not "logically", but because this was the only way it could be done in the circumstances.

This means that to reach the light-sensitive pigment, light entering the eye has first to pass through the (transparent) layers of nerve cells etc.

After the light hits the pigment layer, the nerve impulse then travels "backwards" to get to the optic nerve and then out to the brain.

Does that make sense? Its described on the wikipaedia page on the retina if you need more detail
(edited 7 years ago)
Original post by OxFossil
It is shown that way round because - bizarrely - the anatomy of the retina is actually back-to-front in its structure. It's one of those quirks that shows us how the eye evolved - not "logically", but because this was the only way it could be done in the circumstances.

This means that to reach the light-sensitive pigment, light entering the eye has first to pass through the (transparent) layers of nerve cells etc.

After the light hits the pigment layer, the nerve impulse then travels "backwards" to get to the optic nerve and then out to the brain.

Does that make sense? Its described on the wikipaedia page on the retina if you need more detail


I don't understand... surely the optic nerve is right at the back...
Original post by Uni12345678
I don't understand... surely the optic nerve is right at the back...


Mostly, but it starts where the ganglions from the retina come together. The ganglions carrying the impulse from the rods and cones are routed over the top of the retina (they are almost completely transparent, so they don't interrupt the light hitting the rods and cones beneath them) until they join together at a single point. This is where the optic nerve starts. The nerves then have to "dive" through the layers of rods and cones etc to exit the eye and travel towards the brain proper. At the point where the optic nerve starts in the eye, there are all these axons plus blood vessels coming together and therefore no room for any rods and cones underneath. That's where we have a blind spot in our field of vision.
(edited 7 years ago)
Does this picture help?F16-01 Human eye.jpg

See how the "nerve fiber layer" lies on the inside of the eye?
(edited 7 years ago)
Original post by OxFossil
Does this picture help?F16-01 Human eye.jpg

See how the "nerve fiber layer" lies on the inside of the eye?


Okay so I kinda understand, basically the bipolar cells and optic nerve are on top of the rods and cones and light has to go through them to get to the rods and cones. It then travels backwards through those bipolar cells and optic nerve and then dive back in to the eye? So it goes one way, then the other, and then back the same way?
Original post by Uni12345678
Okay so I kinda understand, basically the bipolar cells and optic nerve are on top of the rods and cones and light has to go through them to get to the rods and cones. It then travels backwards through those bipolar cells and optic nerve and then dive back in to the eye? So it goes one way, then the other, and then back the same way?


Yep! Because it's so messy, most diagrams show the retina as if it is made up of rods and cones with the nerves going "out the back" and down to the optic nerve. Which is simpler for teaching purposes, but wrong.

The AQA diagram shows it as it really is - which is confusing!
(edited 7 years ago)

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