Light is taken to mean the electromagnetic radiation that the human eye can perceive. This is the radiation between the wavelengths of approximately 380 nm and 780 nm. There are no precise limits for the spectral range of visible radiation as they depend on the amount of radiated power reaching the retina and on the sensitivity of the eye of the observer.
To perceive light, the human eye contains two types of light-sensitive receptors:
- The cones that enable us to see in colour ("photopic vision"), with maximum sensitivity to light in the yellow-green range of the spectrum at a wavelength of 555 nm (daytime curve V (l)).
- The highly sensitive rods that enable us to see in black and white ("night-time vision"), with maximum sensitivity to light in the green range of the spectrum at a wavelength of l = 507 nm (night-time curve and V' (l)).