For an optical system in air, the effective focal length ("f" and "f′") gives the distance from the front and rear principal planes (H and H′) to the corresponding focal points (F and F′). If the surrounding medium is not air, then the distance is multiplied by the refractive index of the medium ("n" is the refractive index of the substance from which the lens itself is made; "n"1 is the refractive index of any medium in front of the lens; "n"2 is that of any medium in back of it). Some authors call these distances the front/rear focal "lengths", distinguishing them from the front/rear focal "distances", defined above.