A light with wavelength 450 nm is gradually brightened. Which system and photoreceptor detect it first, and how does the color change as it becomes brighter?

• A. Scotopic/rods; colorless to blue
• B. Scotopic/rods; blue to color
• C. Photopic/cones; colorless to green
• D. Photopic/cones; blue to red

Answer: (A)

The important idea is how vision shifts from rod-based, low-light detection to cone-based color perception as light levels rise. In dim conditions, vision is scotopic and driven by rods, which are very sensitive but do not convey color information. A 450 nm blue light can be detected by rods even when it’s faint, but you don’t perceive color yet. As brightness increases enough to engage the cone system (photopic vision), the blue-sensitive cones respond strongly to 450 nm light, producing a blue percept. So the first detection is by the rods, and the color changes from colorless to blue as the light gets brighter.