In the visible region the dispersive powers and the mean angular deviations for crown and flint glass prisms are $\omega, \omega^{\prime}$ and $d, d^{\prime}$ respectively. The condition of getting dispersion with zero deviation, when the two prisms are combined is

• A

  $\sqrt{\omega d}+\sqrt{\omega^{\prime} d^{\prime \prime}}=0$
• B

  $\omega^{\prime} d+\operatorname{\omega d}^{\prime \prime}=0$
• Correct answer
  C

  $\cos +\omega^{\prime} d^{\prime}=0$
• D

  $(\omega d)^2+\left(\omega^{\prime} d^y\right)^2=0$

Hence, the correct answer is option: 'C'.