What wavelength is commonly used as a practical threshold between scotopic and photopic sensitivity?

• A. 550 nm
• B. 500 nm
• C. 650 nm
• D. 700 nm

Answer: (C)

Two groups of photoreceptors shape how we see under different light levels: rods drive scotopic vision in dim lighting and cones drive photopic vision in bright lighting, each with its own spectral sensitivity peak. The practical boundary between these two modes is often taken at about 650 nm. Rods are very insensitive to red light, so their contribution drops off steeply as you move toward longer wavelengths. At around 650 nm, cones—especially the long-wavelength (L) cones—still respond, while rod vision is essentially negligible. That combination provides a convenient, widely used dividing line: wavelengths around 650 nm are seen mainly through the cone system when light levels are sufficient, marking the switch from rod-dominated (scotopic) to cone-dominated (photopic) sensitivity.

So 650 nm is the best choice because it represents that practical separation point between rod-driven and cone-driven vision. The other options don’t serve as a consistent boundary: 550 nm sits near the photopic peak and would blur the division; 500 nm is toward the rod peak and would be less representative of the boundary; 700 nm is well into red light where cones still respond but isn’t the conventional threshold used in vision science.