Dawkins gave a concise explanation of the evolution of the eye here as well:
This seems like a very basic condition, and that going from 0 to 'just a single sheet of light sensitive cells' is almost nothing. But of course that is not the case. Before that you would need (+++):
1) Photoreceptor Proteins 2) Functional nervous system or signal processing pathways 3) A machinery to translate the absorption of light into an electrical signal 4) A system for coordinating these signal with other parts of the organism . . .
If we're going to study an evolutionary process, we have to pick a starting point. "How an eye evolves from a single sheet of light sensitive cells" is particularly relevant because creationists like to claim that "half an eye isn't useful, so it couldn't have evolved".
"How does a single cell evolve light sensitivity" is not a particularly hot button topic. Creationists don't drone on about plants.
The other flavor are the ones saying "wait a second microbiology is really complex, I'm not sure the standard evolutionary process cuts it here."
Saying, "well let's just assume microbiology isn't a problem", isn't going to be compelling to them.
It is an interesting endeavor, but with respect to the whole creationism vs evolution discourse it feels like a complete waste of time.
You portray such a +++ animal as simple and obvious, but indeed in fact you would need (++++++):
1) adherins and cadherins to bind this sheet together 2) an organelle to produce and regulate the cellular proteins 3) a method to encode those proteins 4) a method to replicate and propagate that encoding so that new proteins can be encoded 5) start and stop codons and a method of duplication or recombination errors to allow for new copies of protein encodings to be created separately from old copies...
Iirc, some snakes have some heat sensing cells arranged in pits to give them a directional heat sense. If you did that with visible light instead of IR those pits would be what halfway to an eye would look like
Like light sensitive cells these would need a similar 1 - 4 +++, so the point would really be the same.
A cell sensing light can use the same type of nerves etc as other sensory cells, so there is no need to explain how a cell sensing light + a nerve + a central nervous system evolves in one step.
If you wonder how a nerve could evolve for example, that is a different question. If you have two cells and they can exchange chemical signals, which is useful in itself, you have the start of a nerve.
If you're going to make that objection, you might as well head straight up the chain and aim for the biggest example of all: how does the first self-replicating molecule appear?
For exmaple, for #3, you say electrical signal. But cells normally use chemical, not electrical signals.
For #1- nope, just need an enzyme or other thing that makes a pigment molecule, and something that can detect a chemical (re-use of an existing protein)
For #2- no nervous system, and cells already have many internal signal processing systems.
For #4- those already existed.
Biology mostly just copies and re-uses systems that already exist. It's still incredible there was a path of mostly random events that led to fully formed eyes, though.