and similarly why is blue light usually looks darker than red or green light in human sight

Blue light appearing darker than red or green light in human vision can be attributed to a few factors related to the human eye's structure and the inherent properties of different colors in the visible spectrum: 1. **Spectral Sensitivity**: The human eye has three types of color-sensitive cone cells; the long-wavelength (L) cones respond to red light, medium-wavelength (M) cones respond to green, and short-wavelength (S) cones respond to blue. Although these cells are capable of detecting blue light, their peak sensitivity is not as high as the M-cones' sensitivity to green light. The peak sensitivity for S-cones occurs around 420-440 nm, which is towards the violet end of the spectrum; however, most sources of "blue" light fall more towards 480 nm, not peaking at the sensitivity maximum. 2. **Luminance Perception**: Blue light has lower luminance compared to green and red light of the same radiant power. Luminance is the measure of human perceived brightness of a light source at any given color temperature. Since human vision is relatively less sensitive to the shorter wavelength (blue) end of the light spectrum, blue light appears more dim to human eyes. 3. **Chromatic Adaptation**: Over periods of time, the human visual system adapts to different color environments. While green tends to be brighter because of the balance of radiant flux and eye sensitivity, blue light, lacking this balance, is often perceived as less intense or darker by comparison in many viewing conditions. 4. **Sky Colors (Natural Environment)**: In natural settings, most people are familiar with the dark appearance of the daytime sky, which is essentially 'blue' due to Rayleigh scattering. This may have an evolutionary effect on how our brains interpret blue as being associated with a dimmer, more distant presence. 5. **Relative Energy per Photon**: Shorter wavelengths like blue have more energy per photon, but they are scattered or absorbed more readily in the atmosphere, requiring more energy to see them, creating the perception that they are less bright or “darker.” 6. **Contrast Perception**: When blue light is compared to red or green light in ordinary situations, it is often seen in contrast, which affects how we perceive its brightness. For example, when placed side by side, the blues might appear less bright because of their lower effective radiance and the lower sensitivity of our rods. Because even minor variations can significantly influence how different colors appear, opinions on color perception can often be quite subjective. Nonetheless, across many studies and under typical conditions, blue light generally looks less bright compared to red and especially green light to the average observer.