The term "light" is not just supposed to mean the light we can see, from red to blue. Visible light is only part of a long scale, analogous to a musical sound scale, in which there are notes both higher and lower than we can hear. The light scale can be described using numbers called frequencies. As the frequency increases, light changes from red to blue to violet to ultraviolet. We can't see ultraviolet light, but it affects our skin, its high intensity can cause burns, it produces vitamin D in living organisms and ozone in the atmosphere. This is also light - just in a different frequency range. If we keep changing the frequency, we will get into X-rays, gamma rays, etc. If we change the frequency in the other direction, we will come from blue light to red light, infrared waves, microwave waves, then to television waves and to radio waves.The monograph is intended for the general reader as an introduction to the world of narrow-spectrum infrared radiation. In a language accessible to the reader, it describes the possibility of a new method of therapy using narrow-spectrum infrared emitters.