As you probably know, light does not spread as direct rays; each “ray” of light consists of fluctuating photons, or quanta. These fluctuations are shaped as repeated waves: if you look at the ray at a super-large scale and “freeze” the time in your mind, you will see that this ray consists of waves, with tops and valleys. The distance between each top or valley defines the wavelength. Our eyes can perceive these waves, but only within a small range of lengths; this range of lengths is what we call color spectrum, or visible spectrum. Each wavelength is perceived by an eye as a specific color; for example, the color red has the longest waves (the least intense), and the violet has the shortest (the most intense). The closest analogue would be sound waves: long waves create low sounds, and short waves, high sounds. Everything that goes beyond the range of the visible spectrum cannot be perceived by an eye, but still exists. Radio waves, x-rays, infrared or ultraviolet waves, and so on--all of them we cannot see, but our devices can.