what is the reason that blue light is looks darker than red or green light in human eyes

Blue light might appear darker to the human eye in comparison to red or green light primarily due to differences in how our eyes perceive different colors, which are influenced by the following factors:

1. Spectral Sensitivity: The human eye has three types of color receptors in the retina, cones, which are sensitive to different ranges of wavelengths corresponding to red, green, and blue portions of the light spectrum. These cones peak at different wavelengths (around 564 nm for red, 534 nm for green, and 420 nm for blue), meaning they have different sensitivities to these colors.

2. Lower Sensitivity at Shorter Wavelengths: While the eye is quite sensitive to green light around 555 nm, the sensitivity drops at both ends of the spectrum, especially for shorter wavelengths associated with blue light. This results in blue light being less effective at stimulating the cones compared to green light, leading to a perception of lower brightness.

3. Melanopsin Role: Apart from the three cone types, there are also rod cells sensitive to low-level light, and another type of cell, intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing melanopsin, responsible for non-image forming functions like pupil constriction and circadian rhythm regulation. The melanopsin is sensitive to blue light (~479 nm peak sensitivity), indicating that blue light can affect our pupils and circadian rhythms but is not as directly linked with brightness perception as green light.

4. Behavioral Factors: As humans, we live in an environment where most natural light sources such as sunlight and the sky appear brightest in the yellow-green range of the spectrum during daytime. This exposure might lead our visual system to favor and be more sensitive to seeing green and red hues as brighter, given their commonness in natural light.

5. Cognitive Aspects: There could also be cognitive and evolutionary reasons why humans perceive particular colors differently. We rely heavily on visual cues for navigation, danger avoidance, and other functions, and these perceptions have been shaped over long evolutionary processes to help survival.

Overall, blue light appears darker because human eyes are less sensitive to shorter wavelengths, and our visual system and brain are optimized to respond more brightly to colors such as green that play a critical role in our visual perception of the environment.