All objects that we observe are focused sharply by the lens system of the eye on its retina. The retina which is located at the back side of the eye has light sensitive organs which measure the visual sensations.
The retina is connected with the optic nerve which conducts the light stimulias sensed by the organs to the optical centre of the brain. According to the theory formulated by Helmholtz the light sensitive organs are of two types rods and cones.
The rods provide brightness sensation and thus perceive objects only in various shades of grey from black to white. The cones that are sensitive to color are broadly in three different groups. One set of cones detects the presence of blue color in the object focused on the retina, the second set perceives red color and the third is sensitive to the green range. Each set of cones, may be thought of as being ‘tuned’ to only a small band of frequencies and so absorb energy from a definite range of electromagnetic radiation to convey the sensation of corresponding color or range of color.
The combined relative luminosity curve showing relative sensation of brightness produced by individual spectral colors radiated at a constant energy level is shown in Fig. It will be seen from the plot that the sensitivity of the human eye is greatest for green light, decreasing towards both the red and blue ends of the spectrum.
In fact the maximum is located at about 550 nm, a yellow green, where the spectral energy maximum of sunlight is also located.