why green light is looks much brighter than blue light in human sight

The perception that green light appears brighter than blue light to human eyes is due to several biological and physical factors: 1. **Eye Sensitivity:** The human eye has three types of color receptors, known as cones, which are sensitive to red, green, and blue light. The green cone cells (M-cones) have a peak sensitivity slightly higher than that of blue cone cells (S-cones), making green light appear brighter to our eyes. 2. **Light Wavelength:** Green light has wavelengths that fall closer to the center of the visible spectrum for humans, which coincides with the maximum sensitivity of the eye's photoreceptors. This peak sensitivity improves the efficiency of signal detection, making green hues appear more luminous. 3. **Cone Distribution:** The distribution of cones in the retina isn't equal; there are more M-cones responsive to green wavelengths. Thus, green light stimulates more cones in comparison to blue light, which corresponds to fewer S-cones. 4. **Luminance Perception:** Human vision perceives luminance differently from quantum efficiency (the amount of light actually captured by photoreceptors). For example, the rod cells, which contribute to low-light and color-blind night vision, respond best to yellow-green light, too. Even though they don't significantly contribute to color vision under normal conditions, this might influence perceived brightness. 5. **Adaptation:** The brain's visual system also plays a role in how brightness is perceived. It adjusts constantly to the average intensity of light seen, a process called retinal adaptation. Since natural scenes often have higher green content due to foliage, the visual system might have adaptively heightened the perceived luminance of green. 6. **Psychological Factors:** Humans also judge brightness based on contrast or relative brightness against their surroundings. Greens are common in many natural landscapes, hence might be perceived as being brighter than equally intense but less typical colors like blue. These combined factors make it so that green light typically appears brighter than blue light under similar intensities and conditions to human eyes.