Two lenses have focal lengths $f_1$ and $f_2$ and their dispersive powers are $\omega_1$ and $\omega_2$ respectively. They will together form an achromatic combination if
- A$\omega_1 f_1 = \omega_2 f_2$
- B$\omega_1 f_2 + \omega_2 f_1 = 0$
- C$\omega_1 + f_1 = \omega_2 + f_2$
- D$\omega_1 - f_1 = \omega_2 - f_2$
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