The peak sensitivity for the sensation of brightness occurs at which light colour?

• A. Red
• B. Green-yellow
• C. Blue
• D. Violet

Answer: (B)

Under bright lighting, brightness perception follows the eye’s peak photopic response, which is strongest for green–yellow light around 555 nanometers. This maximum occurs because the L and M cone cells—long- and medium-wavelength cones—respond most efficiently in this region, and the brain combines their signals to produce the sensation of brightness. Colors toward the red end, blue, or violet lie away from this peak, so they are perceived as less bright even when their energy is similar. (For very dim lighting, peak sensitivity shifts toward blue-green, but in normal daylight the maximum brightness sensitivity is around 555 nm.)