The visible spectrum, which is the range of electromagnetic radiation that is detectable by the human eye, spans from violet to red. The reason for the apparent gap between violet and red is due to the way our eyes perceive colors and the specific wavelengths of light that are visible to us.
The visible spectrum is a continuous range of colors, with each color corresponding to a specific range of wavelengths. At one end of the visible spectrum is violet, which has the shortest wavelength, while at the other end is red, which has the longest wavelength. Our eyes have receptors called cones that are sensitive to different wavelengths of light. Specifically, there are three types of cones that respond to short, medium, and long wavelengths, roughly corresponding to blue, green, and red light, respectively.
While violet and red are at opposite ends of the visible spectrum, it's important to note that the spectrum is actually continuous and doesn't have a physical gap. However, there is a physiological phenomenon that affects our perception of colors, known as color perception or color vision.
Our eyes have different levels of sensitivity to different wavelengths of light. The sensitivity of our cones gradually changes across the visible spectrum. The cones that are responsible for perceiving red light have their peak sensitivity around the longer wavelengths, while the cones responsible for perceiving blue light have their peak sensitivity around the shorter wavelengths. The cones for perceiving green light fall in between.
Due to the way our eyes and cones are structured, there is a region in the spectrum where our sensitivity to light drops off. This region is often perceived as a gap between violet and red, even though it is a continuous spectrum of light. This gap is sometimes referred to as "purple" or "non-spectral" colors because they don't correspond to a single wavelength of light.
In summary, the apparent gap between violet and red in the visible spectrum is a result of our color perception and the sensitivity of our eyes to different wavelengths of light. While the spectrum is continuous, the way our cones respond to different wavelengths creates a perception of a gap in the colors we see.