Which of the following is the combination that is used in the formation of achromatic lenses?

The correct option is 1 convex and 1 concave lens.

An achromatic lens (or achromat) is an optical system designed to limit the effects of chromatic and spherical aberration. Chromatic aberration occurs because different wavelengths (colors) of light bend by different amounts when passing through a lens, causing them to focus at different points along the optical axis. This results in color fringing around images.

To correct this defect, two lenses made of different types of glass with opposing dispersive properties are combined:
1. A convex lens (converging lens) made of crown glass, which has low dispersion.
2. A concave lens (diverging lens) made of flint glass, which has high dispersion.

By bringing these two lenses together in contact, the dispersion caused by the convex lens is countered and canceled out by the dispersion of the concave lens. Because their focal lengths and dispersive powers are carefully matched, they bring two major wavelengths (typically red and blue light) to the same focal point, resulting in a clear image with minimized color distortion.