He looks directly at the pen and can now see that it is red. Which light-sensitive cells are being used to make
the image now?

Thanks to the rod and cone cells in our eyes, our brains can use light to build images. Recent studies identified a third type of cell that responds to light and dark. Three research groups have now confirmed that melanopsin is the pigment that this cell-type uses, opening possible avenues for treating blind people.

Explanation:

if you answer my question i will answer yours

need to see circuit to get time constant