For a red light at 680 nm, under Purkinje effect, under which lighting condition will it appear brighter?

• A. Scotopic
• B. Both
• C. Photopic
• D. Neither

Answer: (C)

The key idea is how the eye’s sensitivity to different wavelengths changes with lighting level, a phenomenon known as the Purkinje shift. In bright, daytime conditions, vision is governed by cones and peaks around the green-yellow region (about 555 nm). A red light at 680 nm isn’t at that peak, but the cone system still supports relatively brighter perception of longer wavelengths than the rod system does for very dim light. In dim conditions, rods dominate and their sensitivity shifts toward shorter wavelengths (blue-green), so red light around 680 nm looks much dimmer. Therefore, the red 680 nm light will appear brighter under photopic (well-lit) conditions than under scotopic (low-light) conditions.