Visible colors are perceived when light of specific wavelengths enters our eyes and stimulates the photoreceptor cells in the retina. The range of wavelengths that humans can perceive as colors is approximately 400 to 700 nanometers (nm). This range is commonly referred to as the visible spectrum.
At frequencies below 400 nm, the light is in the ultraviolet (UV) range, which is not visible to the human eye. Similarly, at frequencies above 700 nm, the light is in the infrared (IR) range, also not visible to the human eye.
The reason for this limited range of perception is primarily due to the physiology of our eyes. Our eyes contain cells called cones that are responsible for color vision. These cones are sensitive to different wavelengths of light, with three types of cones being most prevalent: those sensitive to short wavelengths (blue/violet), medium wavelengths (green/yellow), and long wavelengths (red/orange).
When light enters our eyes, it interacts with these cones, and the information is processed by our brain to create the perception of color. However, beyond the range of 400 to 700 nm, the cones are not as sensitive or not sensitive at all to the specific wavelengths, and therefore, we do not perceive them as visible colors.
It's important to note that some animals can perceive light outside of the human visible spectrum. For example, certain birds, insects, and reptiles can see ultraviolet light, while some snakes can detect infrared radiation. Their visual systems have evolved to detect a broader range of wavelengths than humans can perceive.