According to the Young-Helmholtz theory of color vision, the sensation of any color can be achieved by the superposition of pure red, green and blue colors. This fact was proved experimentally and indicates that in the eye there are three types of receptors, which are sensitive separately to red, green and blue light. These receptors are excited in proportions that correspond to the color of the visible light. Red light excites only the red light receptors, green light excites the receptors responsible for green light, and blue light receptors of blue light. If all receptors are excited to an equal degree, we have the sensation of white light, and if the receptors are not excited, the sensation of darkness. For this reason, the overlapped spots of the red, green and blue light shown in the figure look like a white spot. Additionally, the superposition of red and blue lights appears magenta, superposition of the green and blue lights appears cyan, and superposition of red and green colors appears as a yellow color.

This figure shows the relative spectral sensitivity of the eye in daylight and twilight. Maximal sensitivity to the daylight is achieved at a wavelength of 555 nm, while in the twilight the maximal sensitivity of the eye is shifted to a wavelength of 510 nm. The maximal visibility is taken as a unit. The difference of these two curves can be explained by the fact that different receptors of the eye are working in the daylight and twilight. We can see in the figure that the eye can perceive light at a wavelength of about 400 nm to 760 nm. In darkness, the eye adapts so that we can slightly see infrared light at wavelengths up to 950 nm and UV light at wavelengths as low as 300 nm. This visibility spectrum range is limited by natural factors. Our atmosphere's ozone layer absorbs all the rays at wavelengths shorter than 290 nm, while the eye itself radiates strongly in the IR spectrum region. If the sensitivity of the eye in the IR region were the same as to green light, for example, the eye's own IR radiation would interfere with the visible IR light.