The human visual system is sensitive to a specific range of wavelengths known as the visible spectrum, which spans from approximately 360 nanometers (nm) to 750 nm. This range corresponds to the colors we perceive, from violet to red.
The reason humans cannot see ultraviolet (UV) light, which falls below the 360 nm threshold, is primarily due to the anatomy of our eyes and the photoreceptor cells present in our retinas. Our eyes contain photoreceptor cells called cones, which are responsible for color vision. Cones have different types that are sensitive to specific wavelengths within the visible spectrum.
The three types of cones are most sensitive to red, green, and blue wavelengths, respectively. The combined responses of these cones allow us to perceive a wide range of colors within the visible spectrum. However, our cones are not sensitive to the shorter wavelengths of UV light.
Furthermore, to protect the delicate structures of the eye, the lens and other structures in the front of the eye absorb and filter out much of the UV light that enters, preventing it from reaching the retina.
Although humans cannot see UV light naturally, some animals have adaptations that allow them to perceive UV light. For example, certain birds, insects, and some other mammals have UV-sensitive photoreceptor cells, extending their visual range into the UV portion of the spectrum.
It is worth noting that while humans cannot see UV light, we can indirectly detect its presence or effects through technological devices or materials that can interact with UV radiation, such as UV cameras, fluorescent materials, or UV-blocking sunglasses.