Which cone type has its peak absorption at 557 nm?

The cone type that has its peak absorption at 557 nm is the M cone, which is specifically designed to absorb light in the green portion of the spectrum. The M cone, or middle-wavelength cone, is one of the three types of photoreceptors in the human retina, along with S cones (short-wavelength) and L cones (long-wavelength). While S cones peak around 420 nm and L cones peak at around 560 nm, it is the M cone that exhibits peak absorption at approximately 535-550 nm, which corresponds closely with the 557 nm value mentioned in your question.

This peak absorption at the green wavelengths allows the M cones to play a crucial role in color discrimination and visual perception, particularly in differentiating between shades of green and red. The response of the M cones combined with the outputs from the other cone types provides the brain with the necessary signals to process color vision.