What is the spectral sensitivity peak of M cone?

The spectral sensitivity peak of the M (medium-wavelength) cone is indeed at 520 nm. This peak reflects the wavelength of light at which the M cones are most responsive, making them integral to color perception, particularly in the green part of the spectrum.

M cones, along with S (short-wavelength) and L (long-wavelength) cones, form the basis of trichromatic color vision. Their sensitivity to different wavelengths allows the human eye to perceive a broad range of colors. The optimal sensitivity of M cones being at 520 nm is why this choice is recognized as correct in relation to visual perception.