Which retinal system is more sensitive to wavelengths shorter than 650 nm?

• A. Photopic
• B. Scotopic
• C. Purkinje shift
• D. Stiles Crawford effect

Answer: (B)

In dim lighting, vision is dominated by the scotopic system, which is rod-mediated. Rods are most sensitive to shorter wavelengths, with peak sensitivity in the blue-green region (about 500 nm), and they retain good responsiveness to light that’s shorter than red. Because the question asks about wavelengths shorter than 650 nm, this rod-based system is the more sensitive pathway in that range. Under brighter conditions, cones take over (photopic vision), which emphasizes different wavelengths and isn’t as tuned to the short-wavelength region when lights are low. The Purkinje shift is the phenomenon where peak sensitivity moves toward shorter wavelengths as illumination drops, supporting why scotopic vision is more responsive to those wavelengths. The other terms describe phenomena or the other (cone) system, not the retinal system being asked about.