Which cells in the retina encounter light last?

• A. Bipolar cells
• B. Photoreceptors
• C. Ganglion cells
• D. Horizontal cells

Answer: (B)

Light entering the eye has to pass through several retinal layers before a signal is generated. Photoreceptors sit at the back of the retina and contain light-absorbing photopigments in their outer segments, facing the choroid. Thus photons travel through the intervening layers—where cells like ganglion, bipolar, and horizontal cells reside—before reaching the photoreceptors. Those other cells respond to signals from photoreceptors rather than directly detecting light, so they are encountered by light earlier in the path. Because photoreceptors are the last cells the light reaches to be detected and converted into an electrical signal, they encounter light last.