Two incident monochromatic waves whose wavelengths differ by a small amount $d\lambda$ are separated angularly at $\theta$ and $\theta + d\theta$ with respect to the incident ray. The dispersive power is given by

• A
  $d\theta / \theta$
• B
  $d\theta / d\lambda$
• C
  $d\lambda / \lambda$
• D
  $\lambda (d\lambda / d\theta)$