In practice, the ideal antireflective coating does not exist. It is difficult to find a coating of the proper index of refraction, and the single-layer antireflective coating is of the proper thickness for only one wavelength of light. Thus, even at the design wavelength, reflections cannot be completely eliminated, and wavelengths other than the design wavelength are somewhat reflected. The thickness of a single-layer coating usually is chosen to be optimal for a wavelength in the middle of the visible spectrum (in the green), so more red and blue light than green light is reflected (Fig. 18). Therefore, reflections from a single-layer coating usually are purple, although the total reflection from the lens surface is decreased.