why does green light looks more brighter than red or blue light in human eyes

The perception of green light as appearing brighter than red or blue light to the human eye is primarily due to the spectral sensitivity of human photoreceptors, specifically the cones in the retina:

1. Spectral Sensitivity: The human eye has three types of color receptors, or cones: S-cones (for short wavelengths, typically perceived as blue), M-cones (for medium wavelengths, typically perceived as green), and L-cones (for long wavelengths, typically perceived as red). Each type of cone is most sensitive to different wavelengths of light.

2. Peak Sensitivity: The M-cones are particularly sensitive to light in the green portion of the spectrum, specifically at around 555 nanometers. This is where their peak sensitivity lies. When light matches this frequency closely, it appears brighter because it triggers a higher response from the M-cones.

3. Luminance Perception: Green light is more efficiently detected by these M-cones. While S-cones and L-cones also play roles in color perception, they are less sensitive to light in absolute terms compared to M-cones. This difference in sensitivity can make green appear about twice as bright to the human eye as blue or red light under similar conditions.

4. Brightness vs. Intensity: "Brightness" isn't just about the intensity of the light but also how well it's received by our visual system. Since green light aligns well with how well our eyes naturally detect light, it can seem brighter.

5. Visual Processing: Finally, the processing of colors in the brain contributes to how we perceive light. Even if a red or blue light is of high intensity, if it doesn't trigger the same response in the cone cells due to their lower sensitivity at those wavelengths, it will not appear as "bright."

It is worth noting that factors such as ambient lighting, individual differences in vision, and the specific intensity and wavelength of the light can also affect how we perceive brightness.