Light at which wavelength stimulates all 3 cone types?

The correct answer relates to how different wavelengths of light correspond to the stimulation of the three types of cones in the human eye, which are sensitive to different parts of the visible spectrum. Each cone type—S (short), M (medium), and L (long)—is stimulated by light of specific wavelengths.

In this case, the wavelength of 475 nm falls within the blue range of the visible spectrum and is particularly effective in stimulating both S and M cones. While it does engage these two types effectively, it does not stimulate all three cone types equally. Instead, the wavelength that stimulates all three cone types to some degree would be closer to the middle of the spectrum, around 555 nm, which is often associated with the peak sensitivity of the human eye under daylight conditions.

Therefore, while the chosen answer highlights a competitive wavelength for certain cones, it does not encapsulate the complete stimulation of all three cone types. The wavelength that effectively engages all three types is 555 nm, leading to the perception of white or achromatic light as all cones are activated sufficiently to reflect the entire visible spectrum. Understanding this point is key for grasping how our color vision is synthesized through the combination of signals from each cone type based on different wavelengths of light.