The phenomenon of colors appearing brighter as wavelengths get shorter is primarily due to the way our visual system perceives light and the characteristics of light itself.
Light consists of electromagnetic waves that span a range of wavelengths, with shorter wavelengths corresponding to higher frequencies and higher energy. The visible spectrum, the range of wavelengths our eyes can detect, extends from approximately 400 to 700 nanometers (nm). Within this range, the shortest visible wavelengths appear violet, while the longest appear red.
Our eyes contain photoreceptor cells called cones, which are responsible for color vision. Cones are sensitive to different wavelengths of light, with three types of cones primarily responsible for detecting red, green, and blue light. When light enters our eyes, these cones are stimulated to varying degrees depending on the wavelengths present in the incoming light.
The perception of brightness is influenced by the response of these cones. In general, cones are more sensitive to light of medium wavelengths, which corresponds to green light. As wavelengths move away from the medium range, sensitivity decreases. This means that cones are relatively less sensitive to longer (red) and shorter (blue/violet) wavelengths.
Now, when shorter wavelengths, such as blue and violet, are present, they are relatively less attenuated or absorbed by the eye's internal structures (e.g., lens and cornea). Therefore, more of the blue and violet light reaches the cones, stimulating them more strongly. This heightened stimulation is interpreted by our brain as increased brightness or intensity.
Conversely, longer wavelengths like red are more absorbed by the eye's internal structures, resulting in reduced light reaching the cones. As a result, red light appears relatively less bright compared to blue and violet light.
It's important to note that the perception of color and brightness can also be influenced by other factors, such as the overall intensity of the light, surrounding context, and individual differences in color perception. However, the relationship between wavelength and perceived brightness described above contributes to the general observation that shorter wavelengths, such as blue and violet, appear brighter to our eyes.