The Achromatic lens is composed of convex and concave lenses made of two glasses with different optical properties. Convex lenses are commonly made of crown glass with low refractive index and high dispersion ability, while concave lenses are made of flint glass with high refractive index and low dispersion ability. In general, Achromatic lens can only overlap the image points of the two main colored lights (see figure), so as to eliminate the chromatic aberration of the two colored lights. For example, for the Optometry aid instrument, the color difference of C line (red light with a wavelength of 656.3 nm) and F line (blue light with a wavelength of 486.1 nm) can be eliminated in view of direct observation with the eyes; For photographic lenses, from a photochemical perspective, the color difference between the D line (yellow light with a wavelength of 589.3 nanometers) and the G line (purple light with a wavelength of 430.8 nanometers) can be eliminated. Although there is still residual color difference for other colored lights, it has little impact on general applications and can be considered as basically eliminating color difference.